I'm using UART.write(uartNo, 'AT\r\n'); to execute the command AT in a SIM800l module but I'm getting back AT\x0d\x0a instead of the result of the command according to the documentation.
Did you jhappen to occur to put an oscilloscope or connect to a computer (instead of the GSM module) and see if you are actually sending what you think you are sending and at the correct baudrate ?
Do you know the ESP32 defaults to using UART1 and not UART0 ?
Do you know the SIM800 is sometimes case-sensitive ? (I don't remember exactly when it doesn't like caps) Did you try exactly the same on a serial port on a computer ?