Try each of the following and test after each tip:
1. Update your iPhone to the latest version of iOS
2. Toggle Airplane mode On and Off
3. Try turning iPhone off and then on again.
4. Check for a carrier settings update. Tap Settings > General > About. If an update is available, a prompt will appear.
5. Remove the SIM Card and verify that it is a valid, carrier-manufactured SIM. Also verify that it is not damaged, worn, or modified. Then reinsert it.
If none of these options help, contact your Mobile Carrier and explain the problem to them. It’s likely a carrier issue.