There could be a few causes. The first and most likely would be the signal values, it looks like these are within a poor threshold and could be preventing the device from fully establishing a connection and obtaining an IP address. At the end of this post I’ve pasted a picture which shows signal values and what to look for.
Another possible cause is that AT&T requires your particular SIM card to specify the APN. This would be something to confirm with AT&T but if it does you can configure it through the cellular details page of the Peplink Web Administrator.
Lastly, the current firmware version for Peplink devices is 8.0.0. If you are not already running this version please try upgrading and see if problems persist.