Arduino UNO reset using SIM800L
When the reset pin of Uno is connected to ground, the Uno resets. That is what the reset button does. 

You connected the reset pin of Uno to other device's reset pin. If the reset pin of the other device grounds the reset pin of Uno (grounds or has lower then required voltage), then Uno resets.

The reset pin of a peripheral device is intended to reinitialize the device by program running in microcontroler by setting a connected GPIO pin to required state. 

If you have a library for the device, that library will take care of the GPIO pin controlling the reset of the device.

If you plan to use the device without library, then you should take care of the reset pin in your sketch. Setting the selected pin to OUTPUT and initializing it to required state. 

The usual required state for reset is LOW so the output must be HIGH for the device to work. 

Some tutorials and libraries require connecting the reset pin of gsm800l to a selected Uno GPIO pin with a transistor, and then reset is HIGH and LOW is for normal operation.