Hi There!
I have exactly the same problem, except the fact that I am using the RM32AI as my main mixer, so no additional mixer is in the chain.
I can see the meters on my CS18AI as well as the LEDs on the RM32AI and as described above, the meters also shows up on UCSurface but unfortunately, the result is the same - I can`t grab the signal on the physical monitor outs (XLR 1-16).
I the meantime I have tried almost everything someone can imagine but unfortunately, I am not able to manage to get a signal from monitor outs.
The system is Win10, although I don`t think that the computer platform is involved somehow, so please can anybody help?