Nearly every mixer will only give the main mix as output back to the PC. This is by design, as the purpose of a mixer is to handle all the audio mixing in hardware and give the fully mixed output to speakers or recording devices.
There are mixers that exist that allow you do to multi-track mixing (i.e. SoundCraft Signature 12-MTK). These are usually a lot more expensive. This one in particular will show each stereo pair as an audio input in windows, but these show up pre-fader, so the entire mixing aspect of the hardware is ignored and must be done within OBS.
There are also audio interfaces (the Focusrite Scarlet series is very popular) that have multiple inputs, which will usually allow you to pull each input separately in windows.
That all said, based on what hardware you currently have, thankfully you have subgroup and aux sends.
Assuming your mic is on channel 1, and your guitar is channel 2:
- Channel 1 routed to subgroup
- Channel 2 routed to main and subgroup
- Channel 1 aux 2 turned to mid position (adjust as necessary to avoid clipping)
- Sub 1-2 routed to phones/ctrl room
With this setup, you can connect headphones to your mixer and hear the mic/guitar mix in realtime. The mixer usb output will only include your guitar channel (since the USB can apparently only pulls the main mix for this mixer).
For your mic audio, you will need to route an audio cable from your Aux 2 output to your computer's Line In (you can get a 1/4" to 1/8" cable, or use 1/4"-1/8" adapters however you like). This will let you add the Line In device separately in OBS, and you will still have full fader control from the mixer (Aux 2 specifically... Aux 1 is pre-fader).