... I've heard that you can change the ROM of the phone I've read through some guides and I'm gathering information. I need a T-Mobile ROM for the phone, I'm hesitant to go around downloading one from a random site. Does anyone know any good sites? Also does anyone have any good tips on swapping ROM?
Wherever you heard this is misleading you, stock ROMs are not interchangeable, they are not switchable at a user's whim. The issue is definitely
not like an Install disc for a computer OS where there's an installer that auto-detects the computer hardware and then installs the OS with the necessary hardware drivers. With smartphones, each model is manufactured with its own hardware configuration, and then each ROM is created to match that specific hardware -- the ROM is essentially just a duplicate image of a finalized install. You cannot just pick any ROM you want, flash it, and then your phone magically re-configures its internal components and hardware to match that ROM's integral software.
If you go to
https://willmyphonework.net/ and enter your info, with an AT&T S6 Active the phone will be able to use 2G and 4G but not 3G on T-Mobile's cellular network, with an AT&T S6 only 2G. (You're posting on a S6 Active page but you referenced having a S6). And that's just the cellular radio issue, there are more than likely other differences in hardware configurations between an AT&T and a T-Mobile phone so there are probably a lot of other miscellaneous functionality issues involved (relative any differences with the SoC, GPU, sound chips, sensors, etc., etc.)
But if you want to experiment actually using the incorrect ROM, go here and select the ROM you 'think' you need. Updato is a reliable source for Samsung stock ROMs:
https://updato.com/firmware-archive-select-model
If this is your only or primary phone, note that if your phone gets soft-bricked after intentionally flashing the mis-matched ROM, re-flashing AT&T branded phones to restore them can be difficult as unlike the other major manufacturers, AT&T is pretty reluctant to make stock ROMs publicly available for its products.
I'd recommend you just get a T-Mobile-branded or an unlocked T-Mobile-compatible phone instead of trying to force your current one to work with a different cellular network.
