If you have an AT&T Sim Card, then you need to be in “Advanced” Mode, click the word “Basic” in the web page header and then go to Network>Mobile and set the Service Mode to “4G (LTE) Only” and then click the “Save+Apply” button at the bottom of the page….

 

After the new settings have been applied, go to Network>Interfaces and then click on the “Pencil” Edit button of either MOB1S1A1 (SIM1) or MOB1S2A1 (SIM2) depending upon which SIM slot the AT&T SIM is in….

 

In the Pop-Up settings window, turn off the Auto APN switch and some new boxes will appear. Select “Custom” from the drop down APN list and then type “Broadband” in the Customer APN box exactly as written with a capital B and then click the “Save+Apply” button at the bottom of the page…

After the settings have been applied, go to Status>Overview and check in the Modem panel that you are connected.

Now try going to another website to check you have internet.