Hey, sorry to hear about the issue.
Unfortunately, UltraTap in standalone mode defaults to match the attached interface, up to 2 input channels. This is why you are seeing a stereo input with your interface. You'd have to use a Y-cable to feed channels 1/2. At the moment there are no plans to add a sum to mono button.
While standalone mode is provided, we really recommend using the app as AUv3. That's where it shines, as an insert or send in a larger chain, where the host provides all the routing options you could desire.