We've checked with AT&T from our side, they have confirmed that they've received and registered the provided IMEIs on their end.
Could you please login to the router via WebUI and try to set APN to custom? I'll provide instructions for both legacy and RutOS firmware.
[For devices running with legacy firmware]
In order to find the field in which to set APN, on legacy firmware, please navigate to Network>Mobile and deselect "Auto APN" field and enter one of the custom APNs provided below and save the settings. Note: on legacy firmware after changing the APN you have to restart the mobile connection manually at Status>Network>Restart Connection.
More information is provided on the following wiki page here: https://wiki.teltonika-networks.com/view/RUT950_Network_(legacy_WebUI)
[For devices running with RutOS firmware v7.00 and newer]
On RutOS, the APN settings can be changed at Network>Interfaces>[Edit the mobile interface]>Auto APN. Once Auto APN field is deselected, please enter a custom APN in the "Custom APN" field.
More information regarding where to find this setting on RutOS firmware can be found here: https://wiki.teltonika-networks.com/view/RUT950_Interfaces#General_Setup:_Mobile
A few custom APNs you may try are:
Let me know if this solves your issue.