There is currently an issue regarding the activation of the modem on ATT’s network.
Even though the modem is fully capable of a 4G connection, ATT’s system considers it to be a 3G only modem and as such it will not activate it for usage.
More information below:
https://forums.att.com/conversations/other-phones-devices/ec25a-not-working-att-store-says-its-not-compatible-but-according-to-att-website-it-is/5f0e7a4035f40c43b944c8a6
For now we recommend using a Non IoT Marketplace SIM from ATT.
/Alexandru