I had the same issue with Cubase 10.5 and Catalina (10.5.3) and a UR44. All was working fine before the upgrade to 10.5, and then no input signal at all from my UR44, though it was working on the dspMIX (internal UR44 mixer). I thought would post the solution here.
The resolution I found on this thread from Cypher Audio (thanks). after a) endless searching and b) no response from Steinberg
- Go to Preferences > Security & Privacy > Privacy
- Select Microphone
- in the right hand column there will be a number of Apps with Tickmarks. Cubase 10 was there for me ticked
- (It may be necessary to click the padlock to allow changes)
- Check the box by Cubase 10.5
- (Click the padlock to close if you clicked it to open)
- Close the preferences window and restart cubase
- Hey presto.