Couple of quick tips with ATT

If the SIM card is a pre-paid SIM bought at the store, it might need to be activated - follow the instructions in the SIM kit there.

AT&T uses the “broadband” APN for data access, and make sure to select IPV4/IPV6 - there is no username/password needed, as the SIM card authenticates to the network.

Antenna’s are connected to the Main and Diversity LTE radios, the GNSS is not connected.