Thank you for your help.. I am using a 12V power supply to power the Arduino Nano and I am using Step Down 3A to power the SIM800L. The connection between Arduino and SIM800L is TX and RX only. The board is fully operational. I sent a message to turn on the LED. The board received the message and turned on the LED.. but for the source, when I unplug the power and then turn it back on again. The code only works when the restart button is pressed.. The programming code does not work after turning off the power and it turns on only when the restart button is pressed.. I want to solve this problem when the power is off and then turn it on. I want the program to be executed without pressing the restart button.
Thank you, can I send you more details?