RAK2247 connection to RAspberry Pi 4 witch file to change SPI port (GPIOs) ChirpStack

Hello all,
I want to connect my RAK2247 to the SPI6 port of my Raspberry Pi 4 witch has 6 SPI ports using ChirpStack. Can you help me to configure this choise of SPI, In which file in ChirpStack directory should I access to change this configuration?
I know that SPI is composed of MISO, MOSI, CLK and CS but I am trying to choose the SPI6 (GPIO’s : 18, 19, 20 and 21) to communicate with my RAK2247 LoRaWan Getaway.
Thank you