Finding the right settings

GEOS / REU does not work

There is some doubt of this is actually correct
If you are wanting to use GEOS or any other program that requires the REU, you may need to double check the address for UltiSID 2. Out of the box, UltiSID 2 was at $DF00, which conflicts with the C64U's built-in REU and causes programs to not see the REU. Instead, I move UltiSID 2 to $D420, and it now works flawlessly.

To get there: C64U Menu - Audio Setup - SID Addressing - UltiSID 2 Address.