Can I use any AT&T SIM, such as a retail SIM?

No, not really.

While any AT&T SIM would work in the Starter Kit to get online, the configuration would be different.  The network for cell phones is different from the one for IoT devices.  So, while the radio bands are the same, the APN is different as well as the SIM management software and data centers.

For use with this portal you specifically need AT&T IoT Data Plan SIMs that we provide.  Using any other SIM will not work with this portal.