Programing Atmega328p paired with SIM900

I have two remote gsm switches. Their developing company has stopped operations and both the devices are now not working. I opened those devices and found onboard atmega328p paired with sim900 gsm module. There is one mini usb port that and its data pins are connected to tx/rx pins of atmega328p. I am trying to program this board with my own code to make it work again. I tried to connect atmega with ch340 usb to ttl adapter but not able to program. Can anybody please help me to reprogram this board.

 

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