Sadly, it may be that you have a faulty unit. But since you noted you were unaware of the dspmixfx app, I wonder if there may be something else in your setup which isn’t quite right. In order to assist further I would need you to outline your entire setup verifying all physical connections and routing within your DAW (Reaper) and provide screenshots so it can be verified. I also asked about your headphones. Please let us know the make and model.