[Q] G900T Mobile Data not working on ATT topic

My friend purchased a G900T that was broken for parts, which needed a new screen. Upon replacing the screen, the phone was functional, but he discovered that the IMEI number had been blacklisted on T-Mobile's network. We unlocked the phone to use it on ATT, and the device works fine on the ATT network, with the exception of mobile data. No matter what steps we have taken, putting in custom APN settings, we have not been able to get data to function.

Does anybody have any suggestions as to what might be causing these issues?

Thanks a lot in advance! :good: