Rover only, zero RC channels detected

It’s possible I am missing an obvious parameter setting.

When I shift my pixhawk/px4 over to the Rover platform, the radio will not calibrate and the error “Detected zero radio channels” appears if calibration is attempted. On the Radio tab under Vehicle Setup, the channel monitor section is blank.

Calibration works flawlessly with the quadcopter airframe. All channels are detected correctly. It works for the handful of other airframes I have tried… except Rover. This is all with the same radio, receiver, and wiring.

Any advice would be appreciated.

Firmware version 1.9.2.
Airframe type: Rover
Vehicle: Generic, Axial, or Traxxas – same error for all three.
Radio is Turnigy 9xr-Pro
Reciever Frsky D8R-XP set to CPPM output mode
QGroundControl version 3.5

FWIW, with the Rover platform, all relevant sensors are recognized and calibrated correctly. Just the RC isn’t showing up.