If a message appears on your mobile that says “not registered on network”, it could be caused by a hardware or software problem.
However, in many cases, it is due to simple problems with quick solutions, which can make your mobile work again. It is a fairly common problem with Android devices across all brands and operators.
Most of the time, the user simply needs to restart the device or activate and deactivate airplane mode again, and this usually solves the problem of the unavailable mobile network.
However, sometimes it is necessary to re-register the phone and reconnect it with the operator.

Check Here for Solution: not registered on network
Check Here for Solution: phone not registered on network