ATT Prepaid Data connection issues

My Cat 18 is having problems with my ATT Prepaid Data plan. The cellular data keeps disconnecting about every three minutes which 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. I’ve also noticed that when on the ATT data plan my Ring alarm errors out saying “your attempt to connect to data service was unsuccessful, please log in or try later”.