After reading countless posts on the subject I can't seem to find a clearly definitive answer so I'm giving it a shot here...
If you buy a Telus Desire, SIM unlock it and then put in the correct carrier APN information will the phone correctly jump back and forth between EDGE and 3G on Rogers' network? Half the posts I've read said the phone operates exactly as if it were a Rogers branded phone in the first place and everything works perfectly, the other half say the phone can't transmit/receive data once you're outside of Rogers' 3G coverage as it falls back to looking for a slower connection on CDMA instead of EDGE. I really like this phone but I have a Rogers contract and would need the phone to work correctly on all freq bands for it to be viable for me to own one. I also have no desire to take the more extreme route I've read about which is the whole root/flash new rom method.
Also, is there an easy way to update the phone to Froyo without, again, having to go some convoluted route like goldcard/update with WWE patch? In other words, has anyone with a SIM unlocked Telus Desire seen an OTA update to Froyo showing up anywhere? Thanks.
I don't have the Telus Desire but my understanding is: the Telus Desire is willing to connect to voice/SMS over 2G but fails to connect to data. Using an unbranded firmware fixes it.
To address a few points:
- CDMA is irrelevant. Telus phones can either run on their CDMA or HSPA network but none are capable of both. This is because Qualcomm will not allow phone manufacturers to build a phone with their CDMA chip if it also supports North American UMTS frequencies. Thus all the "world phones" sold for US/Canada CDMA networks only support Euro 3G bands.
Boodah said:
I really like this phone but I have a Rogers contract and would need the phone to work correctly on all freq bands for it to be viable for me to own one. I also have no desire to take the more extreme route I've read about which is the whole root/flash new rom method.
Also, is there an easy way to update the phone to Froyo without, again, having to go some convoluted route like goldcard/update with WWE patch? In other words, has anyone with a SIM unlocked Telus Desire seen an OTA update to Froyo showing up anywhere? Thanks.
Click to expand...
Click to collapse
There is no Telus Froyo OTA. If you want to shove on the international unbranded Froyo yourself you can do so using the RUU method but you will never be able to root nor return the phone Telus firmware without some very extreme measures.
If you want to be able to revert to Telus stock firmware - which is currently NOT to my knowledge available as a RUU or update.zip - here would be your upgrade route:
- Unrevoked3, root the phone and install latest Clockwork Recovery
- Take a full Nandroid backup from Clockwork Recovery and save that file somewhere safe. It's your only way home.
- Locate on the forums: stock HTC pre-rooted Froyo in update.zip format
- Flash that firmware and now you have both Froyo and the EDGE fix
If you ever need to get back to Telus stock (or as close to stock as possible), you can restore the Nandroid backup. You'll now have a rooted Telus firmware.
What I don't know is:
- Whether there is a way to backup/restore the stock Telus recovery
- Exactly which way a Telus user would unroot.
Your understanding seems to be similar to mine at this point, that Telus specifically blocked whatever it is that it takes for the phone to connect at the slower data speed on Rogers' network. But again, I'm seeing contradictory posts about this which is why I was asking in the hope that someone who DOES have the Telus Desire and is using it on Rogers could confirm it. Thanks for the input, though.
the reson why the telus 2.1 rom does not connect to edge is probably because they never had it set up to do so seeing as telus does not offer any edge/2g network.
POSTED IN ANY QUESTION THREAD - CAN DELETE THIS AND PLEASE REPLY TO MY Q THERE THANKS - SAME Q JUST FIGURED EASIER TO POST IT THERE
Have a stock non-unlocked (no unlock) at&t phone with 4.2 android.
Could I use the 4.3 "no-knox" modem.bin I see in these forums that's for t-mobile and just ODIN it over top of the at&t 4.2 modem (mdl modem for at&t that came on the phone when it was purchased) and then use CWM to install a t-mobile rom and have the phone work under t-mobile?
drtechnology said:
POSTED IN ANY QUESTION THREAD - CAN DELETE THIS AND PLEASE REPLY TO MY Q THERE THANKS - SAME Q JUST FIGURED EASIER TO POST IT THERE
Have a stock non-unlocked (no unlock) at&t phone with 4.2 android.
Could I use the 4.3 "no-knox" modem.bin I see in these forums that's for t-mobile and just ODIN it over top of the at&t 4.2 modem (mdl modem for at&t that came on the phone when it was purchased) and then use CWM to install a t-mobile rom and have the phone work under t-mobile?
Click to expand...
Click to collapse
Just because someone else is going to find this thread and start getting ideas...
NO!!!!
No one should EVER do this. Never flash a modem that was not SPECIFICALLY made for your SPECIFIC device.
Skipjacks said:
Just because someone else is going to find this thread and start getting ideas...
NO!!!!
No one should EVER do this. Never flash a modem that was not SPECIFICALLY made for your SPECIFIC device.
Click to expand...
Click to collapse
Ok so tell me please what you mean? Do you mean specific device hardware or ???
Because the only difference I see in the 2 phones, At&t S4 and T-Mobile S4 is all in the software for modem and rom. They are 100% identicle in hardware, even being factory set for bands both carriers use. I would never attempt this on Verizon or Sprint S$'s lol - but I am cinfused other than a different modem bin file and different rom, and different model number since its 2 diff companies, what is the difference?
I am confused.
If I cannot do what I noted, then how does one use an at&t phone on a t-mobile network? I am great at flashing modems, roms, recoveries - but my 1st time switching carriers on an exact same model phone. The phone itself is same model far as samsung makes it - only changes to different model when its branded at&t ot t-mobile and uses that companies rom and mode,m files. But isnt the hardware the exact same both places and couldn't the rom and modem files be swapped to allow it to work on the other one's system?
Please help what I need to do - if I cannot do what I asked about then what?
re: tmobile-att
drtechnology said:
Ok so tell me please what you mean? Do you mean specific device hardware or ???
Because the only difference I see in the 2 phones, At&t S4 and T-Mobile S4 is all in the software for modem and rom. They are 100% identicle in hardware, even being factory set for bands both carriers use. I would never attempt this on Verizon or Sprint S$'s lol - but I am cinfused other than a different modem bin file and different rom, and different model number since its 2 diff companies, what is the difference?
I am confused.
If I cannot do what I noted, then how does one use an at&t phone on a t-mobile network? I am great at flashing modems, roms, recoveries - but my 1st time switching carriers on an exact same model phone. The phone itself is same model far as samsung makes it - only changes to different model when its branded at&t ot t-mobile and uses that companies rom and mode,m files. But isnt the hardware the exact same both places and couldn't the rom and modem files be swapped to allow it to work on the other one's system?
Please help what I need to do - if I cannot do what I asked about then what?
Click to expand...
Click to collapse
If the two phones ATT i337 and Tmobile M919 were as identical as you say they are and even use the same bands then
why is the gsm baseband (modem.bin) and the lte modem (non-hlos.bin) different enough not to be interchangeable?
Not even the STOCK 4.3 firmware or any of the zipfile custom roms here in xda are not identical enough
so we would not need to have two different and separate forums for these two phones.
First thing you need to do is to get your phone on the Tmobile network is to unlock it or have it unlocked.
Then you need to get a tmobile sim card and root the phone if it's not already rooted.
You will also need to flash cf-autoroot to root the phone and then flash a custom recovery like cwm or twrp.
Then last but not least you will need to flash a Tmobile custom rom from the Tmobile M919 forum.
None of the custom roms found here in xda contain modem.bin or hlos-bin images.
Those you already have in your phone will work fine even though they are NOT identical.
The two .bin images are setup so the two networks can establish GSM and LTE communications
between your phone and their towers using different bands and different frequencies.
Good luck.
Misterjunky said:
If the two phones ATT i337 and Tmobile M919 were as identical as you say they are and even use the same bands then
why is the gsm baseband (modem.bin) and the lte modem (non-hlos.bin) different enough not to be interchangeable?
Not even the STOCK 4.3 firmware or any of the zipfile custom roms here in xda are not identical enough
so we would not need to have two different and separate forums for these two phones.
First thing you need to do is to get your phone on the Tmobile network is to unlock it or have it unlocked.
Then you need to get a tmobile sim card and root the phone if it's not already rooted.
You will also need to flash cf-autoroot to root the phone and then flash a custom recovery like cwm or twrp.
Then last but not least you will need to flash a Tmobile custom rom from the Tmobile M919 forum.
None of the custom roms found here in xda contain modem.bin or hlos-bin images.
Those you already have in your phone will work fine even though they are NOT identical.
The two .bin images are setup so the two networks can establish GSM and LTE communications
between your phone and their towers using different bands and different frequencies.
Good luck.
Click to expand...
Click to collapse
Thanks Got her phone working ok. So the key was if unlocked, the at&t phone will work on t-mobile even with an at&t modem file on it I knew the 2 companies could be used on the other's network just did not know the modem file works both ways if unlocked. I assumed the hardware is the same on both phones so just flash a tmobile modem lol - I learned I was wrong thanks for ur help. Years of rooting and never had an issue with using one carrier on another carrier's network, was my 1st time
Thanks a lot for your help. Its working great now!
I bought a used ATT I317 for use on T-mobile network. Just got everything set up, had to flash the J2 modem in order to work around the unlock. Without any further tinkering, it looks like I am getting access to the 4G network via notification (no indication of access to LTE) but getting highest speeds of about 10.25 Mbps down 2.5 up, and lower speeds of 3 Mbps down. I'm wondering if an updated ATT modem would remedy this problem or if I should get a T-mobile based ROM and flash the updated T889 modem to it?
Many of the posts I have come across are older and I believe before the tower integration. Not sure if it is necessary to go further in modem modification or do the AWS band work around. I have my phone rooted and plan to try out different ROMs, especially cyanogen and DittoNote 3, would it work better for me to access the T889 bands from variants of those, or other ROMs?
hexagonation said:
I bought a used ATT I317 for use on T-mobile network. Just got everything set up, had to flash the J2 modem in order to work around the unlock. Without any further tinkering, it looks like I am getting access to the 4G network via notification (no indication of access to LTE) but getting highest speeds of about 10.25 Mbps down 2.5 up, and lower speeds of 3 Mbps down. I'm wondering if an updated ATT modem would remedy this problem or if I should get a T-mobile based ROM and flash the updated T889 modem to it?
Many of the posts I have come across are older and I believe before the tower integration. Not sure if it is necessary to go further in modem modification or do the AWS band work around. I have my phone rooted and plan to try out different ROMs, especially cyanogen and DittoNote 3, would it work better for me to access the T889 bands from variants of those, or other ROMs?
Click to expand...
Click to collapse
yeah first do the latest modem and then the aws mod.
because each time you flash any kind of modem you need to re do the mod
Ah okay thanks. So I should put the latest ATT modem on there and with the AWS mod it should take care of all of my 4G and LTE issues? Then on top of that I should be able to flash any ROM without issue?
Do you happen to have a link to the latest i317 modem?
hexagonation said:
I bought a used ATT I317 for use on T-mobile network. Just got everything set up, had to flash the J2 modem in order to work around the unlock. Without any further tinkering, it looks like I am getting access to the 4G network via notification (no indication of access to LTE) but getting highest speeds of about 10.25 Mbps down 2.5 up, and lower speeds of 3 Mbps down. I'm wondering if an updated ATT modem would remedy this problem or if I should get a T-mobile based ROM and flash the updated T889 modem to it?
Many of the posts I have come across are older and I believe before the tower integration. Not sure if it is necessary to go further in modem modification or do the AWS band work around. I have my phone rooted and plan to try out different ROMs, especially cyanogen and DittoNote 3, would it work better for me to access the T889 bands from variants of those, or other ROMs?
Click to expand...
Click to collapse
yeah first do the latest modem and then the aws mod.
because each time you flash any kind of modem you need to re do the mod
is the UCUBMK6 the current i317 modem? and will the CWM file work on TWRP? (i317-cwm-ucubmk6-modem.zip, from http://forum.xda-developers.com/showthread.php?t=2589891). If its not current, I haven't been able to find whatever is the most up to date modem
hexagonation said:
is the UCUBMK6 the current i317 modem? and will the CWM file work on TWRP? (i317-cwm-ucubmk6-modem.zip, from http://forum.xda-developers.com/showthread.php?t=2589891). If its not current, I haven't been able to find whatever is the most up to date modem
Click to expand...
Click to collapse
yup this the latest and it should work with twrp
So I have am using already a T-Mobile S4 (m919) on At&t's network... I have already rooted it, flashed Hyperdrive, entered the correct APN, and it works basically well...
Here is what I am wondering, my signal strength seems to be not the best and it makes me wonder, is there a modem, baseband I can flash on the T-mobile version of the S4 that could possibly help it work better on the At&t network... and do so with out making my S4 a paper weight?
abovenbeyond said:
So I have am using already a T-Mobile S4 (m919) on At&t's network... I have already rooted it, flashed Hyperdrive, entered the correct APN, and it works basically well...
Here is what I am wondering, my signal strength seems to be not the best and it makes me wonder, is there a modem, baseband I can flash on the T-mobile version of the S4 that could possibly help it work better on the At&t network... and do so with out making my S4 a paper weight?
Click to expand...
Click to collapse
Any M919 modem. MKL is the newest.
abovenbeyond said:
So I have am using already a T-Mobile S4 (m919) on At&t's network... I have already rooted it, flashed Hyperdrive, entered the correct APN, and it works basically well...
Here is what I am wondering, my signal strength seems to be not the best and it makes me wonder, is there a modem, baseband I can flash on the T-mobile version of the S4 that could possibly help it work better on the At&t network... and do so with out making my S4 a paper weight?
Click to expand...
Click to collapse
Glad someone asked that. I looked for a few hours last night and came up empty(been working an off shift so I might have missed it). I am on 4.2.2 MDL and signal is definitely much weaker than my Active on 4.3 . Has anyone compared modems to see which are better?
pahick said:
Glad someone asked that. I looked for a few hours last night and came up empty(been working an off shift so I might have missed it). I am on 4.2.2 MDL and signal is definitely much weaker than my Active on 4.3 . Has anyone compared modems to see which are better?
Click to expand...
Click to collapse
I can provide anecdotal evidence but nothing real...
MK2 is better.
Also, as for which modem to flash; T-Mobile's MK2 would be the one to flash. I've read you can flash other carrier's modems as long as they are the same version, i.e. AT&T MK2 or T-Mobile MK2... but that just doesn't make sense to me and I wouldn't recommend it. The modem you flash is independent of who your carrier is; you go by what brand of phone you have. If you have the M919 then you flash T-Mobile firmwares/modems/etc regardless of which carrier your bill comes from.
abovenbeyond said:
Here is what I am wondering, my signal strength seems to be not the best and it makes me wonder, is there a modem, baseband I can flash on the T-mobile version of the S4 that could possibly help it work better on the At&t network... and do so with out making my S4 a paper weight?
Click to expand...
Click to collapse
I've encountered the same thing with my M919. Switching between Android 4.3 and 4.4.2 modem.bin files made no difference.
I did notice that when I changed my network preference from 3G/2G to 2G-only, I gained about 10 dBa when the handset fell back to GSM-EDGE mode. It made a big difference. I verified that AT&T only uses band II (1900MHz) for GSM and UMTS in my county, so it wasn't because I switched to band V (850MHz) which seems to have better range. What I find annoying is that the handset isn't dropping down to 2G automatically when reception is poor. It'll stick with 3G even when the signal strength is as low as -107 dBa.
No LTE at the moment since my AT&T MNVO doesn't support it.
I'm thinking of buying an M919, carrier unlocked, to use on ATT, so that I can get a proper recovery and have full access to custom ROMs without the safestrap limitations.
Is it safe to say that:
1. If I buy a T-Mobile S4, and it comes carrier unlocked, it will work fine on the ATT network, including 4G / LTE? Lots of data points suggest this but I haven't heard it explicitly stated...
2. Though I could use the phone as identified as an M919, I really want Play Store to see it as an I337 (ATT version), so I can install some ATT specific apps. Is there any problem flashing a build.prop (available in the ATT threads) with the ATT identifiers & APN info, to enable this?
3. Are there any gotchas I should be aware of? Basically, I just want a phone with unlocked bootloader / kernel so I can play. Not too worried about Knox/warranty.
Any insights would be appreciated!
Marc
Frankenscript said:
I'm thinking of buying an M919, carrier unlocked, to use on ATT, so that I can get a proper recovery and have full access to custom ROMs without the safestrap limitations.
Is it safe to say that:
1. If I buy a T-Mobile S4, and it comes carrier unlocked, it will work fine on the ATT network, including 4G / LTE? Lots of data points suggest this but I haven't heard it explicitly stated...
2. Though I could use the phone as identified as an M919, I really want Play Store to see it as an I337 (ATT version), so I can install some ATT specific apps. Is there any problem flashing a build.prop (available in the ATT threads) with the ATT identifiers & APN info, to enable this?
3. Are there any gotchas I should be aware of? Basically, I just want a phone with unlocked bootloader / kernel so I can play. Not too worried about Knox/warranty.
Any insights would be appreciated!
Marc
Click to expand...
Click to collapse
Will the T-Mobile S4 work on the AT&T network; yes it will work if it's carrier unlocked. I purchased my M919 full price out of contract while I was still on AT&T service. Once I carrier-unlocked I was able to pick up AT&T's 4G/LTE and was even able to make use of the AT&T 3G Microcell device which transmits a 3G cell signal and runs over your WiFi. I now have Net10 Wireless with an AT&T SIM card and I still get 4G/LTE when it's around.
Your second question: I do believe that's all that would be required. I'm not 100% sure if just modifying the build.prop would allow you to use the AT&T-specific apps in the Play Store. I never used them to begin with so I never explored whether or not I could make them work on the T-Mobile firmware. Now, I know this involves more than just a build.prop edit, but a 100% sure way to get AT&T Apps working would be to flash a custom ROM based on AT&T firmware. Being that you have a T-Mobile device and are using it on another carrier's network, always remember; when it comes to Odin especially, flash things meant for the T-Mobile M919 ONLY. It doesn't matter that you use AT&T service, your device needs T-Mobile Modems/stock firmwares/etc.
As far as gotchas... not that I can think of. The M919 has an unlocked bootloader and even with KNOX is very easy to root and use custom ROMS on. If you're not concerned with KNOX then you really don't have to take into consideration any special rooting method, no safestrap junk, etc. Without root, just flash a recovery in Odin and flash away.
I know this thread is a bit old, but the situation is very relevant to me. I just won a S4 M919 on eBay which will be delivered tomorrow. T-mobile has agreed to send me an unlock code. My motivation is to install TWRP recovery which I can't do on the i337. I intend to activate it on Cricket (AT&T network). Is it still true that I should stick with firmware, ROMs, and modems designed for the M919? I don't care about AT&T or T-mobile apps so that's not an issue. Basically, I want the best data rates available. Any gotchas that I need to be aware of?
flroots said:
I know this thread is a bit old, but the situation is very relevant to me. I just won a S4 M919 on eBay which will be delivered tomorrow. T-mobile has agreed to send me an unlock code. My motivation is to install TWRP recovery which I can't do on the i337. I intend to activate it on Cricket (AT&T network). Is it still true that I should stick with firmware, ROMs, and modems designed for the M919? I don't care about AT&T or T-mobile apps so that's not an issue. Basically, I want the best data rates available. Any gotchas that I need to be aware of?
Click to expand...
Click to collapse
@flroots
Basically, you're best off bringing the phone to current M919 stock through ODIN. Is that NK2 or has there been a more recent version? I've lost track. But do the entire phone, and in fact as you will see from some threads it is often necessary to flash it twice in succession to get everything to stick. This will bring you to the latest modem. Remember, the modem is two parts: the modem.bin (also known as the gsm modem, which isn't specifically dependent on the bootloader version you have) and the LTE modem (non-hlos.bin, which is dependent on the firmware version, and bad things happen if you mismatch).
Since your phone is out of warranty, there's no downside to going to the latest, even if the phone arrives with 4.2.2 firmware (pre-knox). Tripping the knox bit is only a problem if you would be sending the phone in for warranty service.
I've got the M919 and am using it on ATT, with no problems. I've got the pre-knox firmware (MDL), and will be updating to the latest next time I flash.
Here is an old, but still relevant, thread about my experiences with M919 on ATT.
http://forum.xda-developers.com/galaxy-s4-tmobile/help/using-t-mobile-m919-att-network-t2880349
You will have to configure your APN to match your Cricket service. Sorry, can't help you with those settings but I'm sure they are on XDA somewhere!
Marc
Frankenscript said:
@flroots
Basically, you're best off bringing the phone to current M919 stock through ODIN. Is that NK2 or has there been a more recent version? I've lost track. But do the entire phone, and in fact as you will see from some threads it is often necessary to flash it twice in succession to get everything to stick. This will bring you to the latest modem. Remember, the modem is two parts: the modem.bin (also known as the gsm modem, which isn't specifically dependent on the bootloader version you have) and the LTE modem (non-hlos.bin, which is dependent on the firmware version, and bad things happen if you mismatch).
Since your phone is out of warranty, there's no downside to going to the latest, even if the phone arrives with 4.2.2 firmware (pre-knox). Tripping the knox bit is only a problem if you would be sending the phone in for warranty service.
I've got the M919 and am using it on ATT, with no problems. I've got the pre-knox firmware (MDL), and will be updating to the latest next time I flash.
Here is an old, but still relevant, thread about my experiences with M919 on ATT.
http://forum.xda-developers.com/galaxy-s4-tmobile/help/using-t-mobile-m919-att-network-t2880349
You will have to configure your APN to match your Cricket service. Sorry, can't help you with those settings but I'm sure they are on XDA somewhere!
Marc
Click to expand...
Click to collapse
Thanks a lot for that update. As I understand it, the current firmware is NB4 according to this site:
http://galaxys4root.com/galaxy-s4-stock-firmware/
Not sure what your reference to NK2 is? That doesn't seem to correspond to either 4.2.2 nor 4.4.2? I'm currently on Cricket with a S4 I337 and hope to move the SIM over as soon as my M919 arrives tomorrow.
I believe after NB4 there was NH7 then NK2. Check the firmware thread in the general section for details / links.
Sent from my SGH-M919 using Tapatalk
@flroots
How did this work out for you? I want to do the same thing you said you where going to do.
flroots said:
I know this thread is a bit old, but the situation is very relevant to me. I just won a S4 M919 on eBay which will be delivered tomorrow. T-mobile has agreed to send me an unlock code. My motivation is to install TWRP recovery which I can't do on the i337. I intend to activate it on Cricket (AT&T network). Is it still true that I should stick with firmware, ROMs, and modems designed for the M919? I don't care about AT&T or T-mobile apps so that's not an issue. Basically, I want the best data rates available. Any gotchas that I need to be aware of?
Click to expand...
Click to collapse
Robert417919 said:
@flroots
How did this work out for you? I want to do the same thing you said you where going to do.
Click to expand...
Click to collapse
Robert417919,
I use my SGH-M919 on an AT&T network also. I use the OK3 modem firmware and it seems to work great here. Although I am in Alaska, so top speeds of LTE are only about 6 Mbps where I live. Usually I can only get about 3 Mbps due to the lack of coverage around here.
You can either update to the latest OH3 firmware on your phone, or if you have an old model, with the MDL bootloader, you could also use this guide by lordodin912:
http://forum.xda-developers.com/galaxy-s4-tmobile/general/foh3-modem-bin-mdl-bootloader-t3191094
Also, if you are looking for good roms, you can always check out SlimLP:
http://forum.xda-developers.com/galaxy-s4-tmobile/development/rom-slimlp-jfltetmo-t3377731
Or Resurrection Remix:
http://forum.xda-developers.com/gal...ent/rom-resurrection-lollipop-v5-4-5-t3115155
Hope that helps!
Thanks, this does help out, and another quick question. Are you able to use wifi calling?
AlaskaLinuxUser said:
Robert417919,
I use my SGH-M919 on an AT&T network also. I use the OK3 modem firmware and it seems to work great here. Although I am in Alaska, so top speeds of LTE are only about 6 Mbps where I live. Usually I can only get about 3 Mbps due to the lack of coverage around here.
You can either update to the latest OH3 firmware on your phone, or if you have an old model, with the MDL bootloader, you could also use this guide by lordodin912:
http://forum.xda-developers.com/galaxy-s4-tmobile/general/foh3-modem-bin-mdl-bootloader-t3191094
Also, if you are looking for good roms, you can always check out SlimLP:
http://forum.xda-developers.com/galaxy-s4-tmobile/development/rom-slimlp-jfltetmo-t3377731
Or Resurrection Remix:
http://forum.xda-developers.com/gal...ent/rom-resurrection-lollipop-v5-4-5-t3115155
Hope that helps!
Click to expand...
Click to collapse
Robert417919 said:
Thanks, this does help out, and another quick question. Are you able to use wifi calling?
Click to expand...
Click to collapse
Glad to help, and no, unfortunately not.
-AlaskaLinuxUser
https://thealaskalinuxuser.wordpress.com
I would like to install a T-Mobile Rom or at least the flash the modem files on a Verizon S4. I switched it to T-Mobile prepaid and I feel it would have better service if I did that. Is this possible? I tried flashing basebands and a rom (i don't remember which one) and it didn't work.
You won't get better service. Only thing you can do is update modem for YOUR device, and make sure you have correct apn. Flashing rom is possible I believe but don't know the procedure, try searching the verizon forums
If flashing a T-Mobile rom won't help then I won't even bother. So updating the modem for my device may help reception for another carrier even though the baseband is for Verizon? Unless my understanding of modem/baseband is wrong. I thought that was what told the phone how to communicate with that particular carrier.
It may to some extent, but modems are not interchangeable with these devices