The RAK4631 uses symbolic labes for its I/O Pins on the module and baseboard silk screens. The following table shows the mapping of the RAK4631 GPIO pins to the corresponding Arduino pins and the MCU Port numbers.
There is no usable GPIO pin on any RAK base board except the 'big' baseboard RAK19001 without adding a RAK13002 IO module or a third party IO sensor breakout.