SIM800L interfacing with Arduino nano not working

I’m trying to interface SIM800L core module (Image attached) with Arduino Nano.

SIM800L is powered separately by Vcc 3.7V @1A.

Problem is SIM800L not registering to any network; I’m from Qatar, here there is only two networks.

Here is some results during configuration of SIM800L.
Without SIM card, signal quality command AT+CSQ → +CSQ: 15,0
With SIM card, AT+CSQ → +CSQ: 0,0

And network registration command, AT+CREG=? → +CREG: (0-2)

What is the problem?
Please help…
Thanks in advance.