Pls send me the screenshot of the modem page (where it shows SIM card cannot be registerred.)
I did the AT command again and it changed from 3g to 4g for a few seconds then gave that error again.
See images below.
I should also add that I have to reset the modem after this because it will not reconnect to AT&T.
For a few seconds it would connect as shown here but then 3g/4g symbol is gone and it gives the above error message. I then have to reset the modem because I cannot connect to the internet.
Pls post the Cells info to see if signal is too week.
Pls try to enable IPv6 to see if this matters.