When I set the circuit without sim card, AT commands are working very well. But if the sim card inserted, responses are not coming through serial monitor. My power supply is ~4.2V@3.2Ah (in several forms, they just said that at least 1Ah so ı pluged 3 Ah.) Im using the common red and small sim800L board. And I reduced IO pins to 3.3 v with voltage dividers.
You should check that the connections are right. Maybe it’s a manufacturer error.
Try to check that all the voltages are right when you insert the SIM card.
The sim800 is still on? Or is it powered off when you insert the SIM card ? In my experience all these kinds of problems are related with a power supply error.
I had a similar problem with the sim868 and the problem was that the power supply wasn’t good enough and when I inserted the SIM card PIN the board powered off due the current wasn’t enough.