I have multiple Quectel EC25-AF-FD modems, some connect via AT&T SIM cards and some do not (even with the same SIM cards).
As stated in the title I have multiple Quectel EC25-AF-FD modems, they all work with T-Mobile SIMs. They are supposed to be compatible with AT&T, and some connect with no problem but others do not. I've checked that the hardware versions are the same, the firmware versions are the same, I checked as many configurations as I could find via AT commands to make sure they were set up the same.
What could cause this? Do the modems need to be registered with AT&T by IMEI or ID numbers or something like that?