Default [Q] Can I install a T-Mobile stock ROM on a AT&T unlocked version

Hey guys I need help... I got my wife an unlocked AT&T LG G2. I set up the APN setting and the LTE was working but as soon as I don't have a strong signal the phone drops to gsm and it doesn't give me the options to include other frequencies on settings. I worked with T-Mobile to fix the issue but they don't have a fix. I was adviced to flash a T-Mobile stock ROM is this possible if the build model is different?