1. Check if the SIM card is inserted correctly and locked into place.
2. Check if the SIM card is not damaged.
3. Check if the phone is compatible with the type of SIM card (e.g. nano or micro SIM).
4. Check if your phone’s settings need to be updated.
5. Check if the SIM card’s network settings need to be re-configured.
6. Switch the phone off and back on to reset it.