FrSKY S-Port RSSI not working

Hello there,
I want to build a FPV plane out of my MPX Funray.
My Setup consists of a FrSKY Tandem x20s transmitter in combination with a FrSKY Archer R8 Pro receiver. My FC is a H743-SLIM (flashed Ardupilot 4.4.1 [(4ccbfb1f]).

Sadly I am unable to get Ardupilot to show me the RSSI of the RC radio.
My Config:
SERIAL7_PROTOCOL 10 (I also tested 4 with no impact)

Does anyone know, what I can do to get the RSSI in Ardupilot?