Peplink SIM not registering on network

A SIM that shows inserted but won't register on the carrier network in a Peplink router is often an APN configuration issue rather than a bad SIM — different carriers (and sometimes different plans on the same carrier) need specific APN settings that don't always auto-configure correctly. Check the SIM works in a phone first to rule out a dead SIM, then focus on the APN settings in the router. Confirmed the SIM works elsewhere?