Turn Airplane Mode on, then turn it off
Restart your phone
Check for carrier signal in a different location
Make sure Cellular Data is turned on
Turn Mobile Data off and back on
Remove and reinsert the SIM card
Check that the SIM card is not damaged
Reset Network Settings
Update your phone’s software
Check for carrier outage in your area
Make sure your carrier plan is active
Contact your carrier for account or network issues
Contact Apple Support or your phone manufacturer if the issue continues