OK, so assuming that you're plugging the guitar into the INST jack and not the Mic/Line jack then there's no reason things shouldn't be working fine.
You should expect to have the gain setting on the mic pretty high, but you shouldn't need to do anything too heavy on the instrument input.
So the first thing is to ignore the comment you've quoted from the other forum. The UMC has a built in pre-amp that works automatically on anything plugged into the XLR socket. The same socket works as a line-level input if you plug a jack in there.
So don't worry about that.
There are various places where the issue could be arising so it's important to work through them logically.
Start with the sources. We've established the microphone works because it works over USB. We'll assume for now that the audio output of the mic is correctly wired up well. Unless you have another device (mixer/pa/interface) that you can plug the xlr in and confirm?
Presumably you've also got an amp or something that you can plug your guitar into and confirm that this is also working ok.
Next is cables, never underestimate this step in any problem solving routine! So if you have them, swap out both microphone and guitar cables, or check them using a cable tester or multi-meter.
Then let's look at the signal entering the interface. Each input has two LEDs near the gain knob. The green one should light up when the input is getting a signal in, the red one will light up when the signal is too strong (too 'hot').
For both inputs, set the gain to about 2/3rd and speak into the mic / play your guitar and confirm if you're getting a green light.
If yes, excellent. We'll move on.
If no, try nudging the gain up until you do. If you're at maximum gain with no green light (and you've tested your cables/guitar/mic already) then you probably have a faulty unit.
If you're getting a green and red light, turn down the gain until the red light goes off.
The other way to check the inputs is to plug some headphones into the front and press the Direct Monitor button, this switches the interface to listening to the inputs not the computer output - so you should hear anything that's coming in from the mic or the instrument inputs.
Ok, so if you've got a signal coming (green lights), and you can hear it ok through the headphones with Direct Monitoring engaged, then we need to look at the next step.
I've never used Audacity, but somewhere in the setup/preferences/options will be a menu allowing you to set up your input device or audio device or what have you. You'll need to make sure your inputs and outputs from Audacity are set the UMC22 - if that's not in the options list, you might need to look for ASIO4ALL instead - these are the generic drivers for the audio interface. If you haven't already downloaded these, do. You can get them from here:
https://www.behringer.com/downloads.html
That should set up the sources, the interface, and the DAW ready to take a signal.
At some point round here, make sure your project settings in Audacity are set to record in 24bit. It will probably default to that, but check anyway. Whether it's 44kHz, 48 or 92 is much less important. 24 bit rather than 16 is the important number.
So then add a track, set the input in the track to whichever source you're recording (mic or instrument), prime it for recording and see what kind of levels you get.
This bit is really important - expect your levels to be 'really low'.
When we record at 24bit we get a huge, huge dynamic range. Which means that you don't have to 'push' your inputs hard, you can keep your input at a sensible level so that you don't get any clipping (where your signal is too hot and it distorts nastily).
So what's a sensible level? If your input signal is bouncing around -18dB, with your peaks (the loudest spikes) just hitting -10dB, then you've got a good level. It'll be plenty loud enough when it comes to mixing the track but you don't need to worry about exuberant performances causing any clipping.
BUT this will sound much, much quieter than any other music you play through your system. And that's fine, it's how it's supposed to be. It's easy to raise the volume at the end of the mixing process and bring it back up to 'normal' levels.
There are a couple of things to watch out for though. Some DAWs (and I don't know Audacity) have the default size for the display set so that a healthy signal (bouncing around -18) looks really small in waveform display. Don't let that put you off. You can probably rescale the display somewhere (to make it easier when editing) or just ignore it and accept that a good level looks small on the waveform.
In summary:
Check the sources if you've got anything else to plug into.
Check your cables.
Check your input signal (LEDs and headphones with direct monitoring).
Check your DAW set up.
Check your recording levels.
Hope that all makes sense, just shout if you have any questions.
