Many x750 owners are experiencing the same problem. There is a compatiblity issue between ATT and x750. Nobody knows if this is a temporary or permanent problem. Read this:
https://forum.gl-inet.com/t/x750v2-will-only-connect-to-at-t-at-3g/
and then this:
Executive summary: the x750 situation may be part of their phase out of all 3G service. The x750 may never connect at 4G again, and in feb 2022 it won’t connect at all. If you want to stick with ATT, it appears your only option is to buy a netgear nighthawk hotspot.
Either buy a new router, or switch carriers.