Ur22c audio output issue

Hi, I recently purchased a ur22c and trying to record guitar with it. When I plugged my earphone in the front port, I can only hear a mono-like sound no matter what I played, even a youtube video. For listening to my guitar input, I can hear a normal (stereo) guitar sound when I leave the “mono” button , but the mono issue happens to the guitar input when I pressed down the “mono” button on the ur22c. I’m wondering why this happens and how could I fix it? I connected the audio interface to a MacBook Pro through a USB-c adaptor. Thanks!

I t is not really clear to me what you mean by “mono”, but here goes anyway.
The mono button is used to send the input of one of the inputs to both of the output chasnnels, so that if you plug your guitar into input 2 you will still hear it in both left and right channels. This is assuming that you want to hear your guitar direct, meaning the input knob is not totally clockwise.
If the sound from youtube comes out as “mono”, meaning the exact same sound comes out of both right and left channels, you will have set it up this way somewhere in your computer.
If otho you by “mono” means you hear computer audio from just one of the channels, then it could be you are using a trrs plug (3,5 mm with microphone) with a trs adapter to plug into the 22c, as a lot fo the adapters and headphones don’t get the signals right.
Maybe I misunderstand you completely, then just clarify:)

Thanks for replying! This is very helpful. The issue IS brought by I’m using a headphone with a microphone. The issue is resolved when I use a headphone with no microphone!

Good stuff!