I think that what @Jorge_Canois trying to achieve is just a direct radio connection without passing data through the autopilot, so there is no need to use Mavlink Inject. This feature is needed solely for injecting correction stream into drone telemetry.
Your base station radio is connected over USB, but settings are “UART”. This will not work. If you can’t see USB as an option in app it might be that chip in your radio is not supported by Reach at the moment. Intel provided an old driver in the image and we will address this in the next image update to ensure that all devices are recognized.
Can you connect your base radio over UART as well?
@egor.fedorov posted a temporary fix for the USB not being recognized: