Stereo recording in Reaper from the inputs on my UR22c works fine on W10. I’m monitoring from the headphone out so maybe check connections to the speakers.
On Reaper track:
Recording input - audio or midi
Input stereo UR22c input 1& 2
Record monitoring ON
The UR44c has no mix dial or mono button on the front so maybe that’s controlled in a software control panel.
Worth checking those settings if available.