This error may be due to any of the following reasons:
- SIM card is not installed
- SIM card is inactive
- SIM card is invalid
- SIM card not installed properly
- SIM card is installed in the lower slot instead of upper left slot
- No antenna/wrong antenna attached
- Antenna connector is not secured properly (do not finger tight the antenna connector)
- Antenna not mounted vertically in open space
- APN is not set to match the SIM card cellular service
- UPT is not restarted after inserting a new SIM card.