mobile network and Wi-Fi are blocking
This issue happens for many models.

Let me say a simple scenario

You are in a call, suddenly the signal has gone and call drops. Then even waiting for a long time, the signal is not coming.

This is because, the phone didn't attempt to connect to a network anymore.

The solutions


Turn on airplane mode and after 5-10 seconds, turn off it.
Go to operator selection -> manual -> search and choose the operator, later change to automatic.
As you said, restart the phone.