Insert an active SIM card into the phone
Turn on the phone
Check that airplane mode is off
Enable mobile network or cellular data
Restart the phone
Wait for the phone to detect the network automatically
Select the network manually in mobile network settings if needed
Set network mode to automatic or the correct preferred mode
Verify the SIM is properly seated
Check that the SIM is not damaged or expired
Ensure the account is active and service is not suspended
Confirm the phone is not carrier locked
Update the phone software
Reset network settings if the phone still does not register
Contact the carrier to register or activate the device if required