Hi David. Welcome to justanswer.
Have you tried taking the sim out and then reinserting it?
It is very simple. Check the box in which the phone came. You would have got a sim ejection tool (which looks like a pin). You need to insert the ejection pin into the hole on the SIM card tray to loosen the tray. Pull out the SIM card tray gently from the SIM card tray slot. Remove the SIM card.
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