iPhone Xr not making or receiving calls

I've had this problem before and have had it fixed by an apple genius bar employee but there is no access to genius bar at the moment in my area due to covid. My phone has stopped receiving and making calls. Effectively it does not recognise the carrier network. Last time it happened I followed all the steps recommended on the apple support page including resetting to factory settings. In the end the genius bar employee got me to disconnect from the carrier network and then reconnect. I'm going from memory here, but I think that was what he had me do. It worked at the time and so I am keen to repeat the steps again. Unfortunately I have forgotten the steps involved. Any suggestions?