Please check the possible reason and trouble shooting steps below to solve your issue:
1. The phone is in Airplane mode or there is poor contact between the SIM card and phone, please turn off Airplane mode, and re-install the sim card to restore connection between the SIM card and phone;
2. One of the SIM card slots in dual-SIM phone is disabled, please enable the disabled SIM card slot via Dual-SIM settings to activate the SIM card;
3. The SIM card system is not supported by the phone or the SIM card is faulty, please replace with a SIM card with supported
system or replace with a normal SIM card;
4. The phone serial number (IMEI number) is lost, on dial screen, please enter '*#06#' to view the phone serial number (IMEI number).
If the serial number is lost, bring the phone to a local service center;
5. SIM card deck or tray is deformed, which results in bad contact, please bring the phone to a service center;
6. Phone firmware is faulty, please bring the phone to a service center to fix the software, or download the corresponding firmware from official website and update the phone with it.