I am assuming this is a North American device. Below are some of the first things to look at. If those don't work I would get a USB modem to test the sim in. It rules out carrier/sim being the issue. Then you can go to meraki with that info and get alot farther if needed. I also have heard that on the ATT side you need a static IP and then they will provide the correct APN once you have that setup.   My cellular uplink is stuck at 'Connecting' Built-in Cellular - Ensure the following: The SIM is activated with the PIN disabled or the correct PIN entered. It may be necessary to use an external modem, or work with the cellular provider to have the PIN disabled or the SIM unlocked. There is no external USB modem connected as the MX will prefer an external USB modem to the internal modem, if available.  If a custom APN is needed, ensure it is applied from Cellular section of the Uplink tab on the Security Appliance > Appliance Settings page. The SIM card is fully inserted. ... View more