APN stands for Access Point Name. By configuring the correct APN settings on your device, you ensure that it can establish a proper connection to the internet.
To find what APNs are provisioned to an ICCID, Navigate to Provisioning > select SIMs > enter ICCID > click search then click the ICCID in the Results.
In the Overview tab, on the right-hand side under APNs, the 3 APNs provisioned for the SIM are displayed. Any of the displayed APNs can be used on the device to connect.