Hmm. You SHOULD be able to record that way, if your computer can detect the mic. Off memory and I don’t have Reaper in front of me, but try going to Preferences, and then Audio Devices (with the USB mic plugged in), and see if you can find it as an audio device. If so, select that as your recording device, and then when you go to arm a track to record, from the input dropdown make sure you’ve selected that mic.
(If you’re new to working with a DAW and multiple inputs, I remember this whole thing confused the hell out of me when I bought my first interface and seemed like a needless complication… But the point of it is unlike a standard mic input on a computer, with an outboard interface you can record from a whole bunch of different sources, seperately, all at once. It seems complicated, but only because you have a lot more flexibility on where you record things from and to).
Only thing I’d caution on with the USB mic is for recording a guitar cab, you’re probably going to want a dynamic mic of some sort, and a lot of the USB mics I’ve seen are geared towards the podcast crowd and are condensers. These are awesome for acoustic instruments and for a lot of vocals but less so for high gain amps, where between the greater high frequency response and the greater sensitivity/risk of harm from high sound pressure levels, you usually see dynamics used.