My Cat 18 is having problems with my ATT Prepaid Data plan. The cellular signal keeps disconnecting about every three minutes and forces the modem to get stuck and cycle in the mode to look for a new cellular sim. I have no problems with my T-mobile and Verizon sims. The only way to fix it is to reboot the modem and shortly after ti starts doing it again…only the ATT sim.
May I know the signal for the AT&T connection ? The disconnection can be related to the bad Cell signal for your location as well seen T-Mobile & Verizon have no issue.
For ATT prepaid plan, I would suggest you can contact AT&T whether they do some enforcement whether certain type of device allow to connect to the network.
Some reference Link: