the serial port in this board is the P4 pin header,
upper row is #define SERIAL_PORT_2 2 in marlin
lower row is #define SERIAL_PORT_2 3 in marlin
if you have the tmc2208 module, the upper row is shared with the tmc2208 module, so you can't use as serial communication port
https://github.com/FYSETC/FYSETC-AIO_II/issues/7
also, you can use the USB port as usual #define SERIAL_PORT 1
greetings