SIM card may not be activated.
Check with Network Provider on any Steps or Web UI for Activation.
SIM not inserted correctly or SIM Locked
Put the device in Airplane mode and check that the SIM is properly red. ERROR response usually is an indicator for this problem. You may want to unlock the SIM with code provided by the SIM vendor.
AT+CFUN=4
AT+CPIN?
AT+SQNCCID
AT+CIMI
Antenna may not be properly connected or cable not properly tightened.
Operator mode , APN or band selection is not properly set.
Use AT+SQNCTM command to select to proper operator mode. By default standard mode is used. APN is configurable with AT+CGDCONT command and the band with AT+SQNBANDSEL (see band selection topic)
AT+SQNCTM?
AT+CGDCONT?
AT+SQNBANDSEL?
You may experience poor network conditions
Try to connect at another place where signal could be better. If you are in lab, you may want to check the signal level of the tester, the frequency acceptance of the splitters and the attenuation used. See topic on signal indicators (RSRP, RSRQ, CINR)
AT+SQNMONI=9
Network rejects the device
If you observe +CEREG=3 you may check the result of AT+CEER and take an action depending on the NW reject cause.
AT+CEER
Please refer to the AT Command Reference Manual for more details on this command