I once had the phone drop the phone network and it additionally seemed to indicate in the upper status bar and networking settings page that it was connected to the phone carrier network. However, when attempting to send a message or place a call it would refuse and say that there is no SIM. This survived numerous reboots, but went away after I toggle 4g on and off. I have no idea why this would happen or whether you have the same problem. However, it will to little harm to try this.
you couls also look at the logs, but I have no idea which ones would be pertinent to this issue at hand, perhaps telephony-service-indicator.log?