Hi,
The radio repeater can only display the green (V/UHF) radio info, the red (UHF) radio is not compatible with the instrument.
In order to have the red radio working you only need to set the power knob to the M position, as you did. You can then find the channel frequencies on your kneeboard.
There is a special option to have the repeater directly display the radio frequencies, including the red radio. But this is a game option, it is not realistic.
Thanks.