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The RX2 does not have an input channel on its audio interface; the mic goes from the input to the master, it has no capacity to be captured to the software. The reason you can hear the master audio in your mix / recording is that the audio is mixed within the software itself, then sent to the hardware as the MASTER audio, where it's then mixed with the mic or an aux source to output.
If you want to add the mic to your mix, you need to either capture the audio output from the RX2 into another audio interface, or add a mic connected to your computer (or internal mix) to the recording or OBS as a secondary source.