I have pushed some fixes to enable Serial I2C operation. Following parameters need setting on Ardupilot:
GPS_DRV_OPTIONS 4 # Enables Ublox to operate at 115200
GPS_TYPE/GPS_TYPE2 2 # Defines GPS Type to be Ublox, this is needed to honour above option
GPS_SAVE_CFG 1 # Ensures that baudrate 115200 sticks
Also I2C line needs to be connected as well, and CAN disconnected. I2C is how Here4 will know that it needs to switch to Serial/I2C operation. No other config on Here4 is needed. You should also see status LEDs behaving per Ardupilot status, and also RM3100 mag should appear as well.
The latest firmware build should be used, its available here: fix Serial I2C operation for Here3+ and Here4 · CubePilot/GNSSPeriph-release@9eabf65 · GitHub . Use the file Here4_FW.bin in GNSSPeriph zip under Artifacts.
If all goes well with testing and feedback, I will push a release.