How to load T-Mobile firmware onto AT&T Galaxy S7?

Taking a shot in the dark that one of you tech savvy folks might know how to install the T-Mobie software/firmware onto our AT&T unlocked S7?

We switch to T-Mobile and the S7 was unlocked but for some reason we have a glitch with connecting to the TM data. If I turn Airplane mode on/off the phone will connect to 4GLTE just fine and stay connected as long as I don't reboot the phone. If I reboot, it won't connect to the T-Mobile data. I have to flip on/off Airplane Mode.

I've heard some folks online say putting the T-Mobile firmware onto the phone would solve it, but I'm totally out of my element when it comes to tweaking smartphones.

Anyone ever done something similar? Can anyone point me to some instructions that would be safe to install?

Thoughts? Thanks