Maybe the tablet plan SIM locks some IMEI. X750V2 is identified as IOT devices, not tablet.
But you can try all the apn here North American Carriers, ATT, Verizon, Cricket, T-Mobile Setup and Update 2021
Be sure to set ttl to 65 in the settings.
Use ttyUSB3 as device, not cdc-wdm0
Try lock band, find out which band AT&T is using and lock that band using AT command.