Even if you do not believe, it's very easy and straightforward.
1. Check that TM FX is in submix mode (upper right, blue area)
2. Click the "assign" button in TotalMix FX control room and assign HW outputs for
- "Main Out", usually HW Output AN 1/2
- "Phones 1", HW output AN 7/8.
Note: by this the HW outputs are being moved into the control room on the right side so that you can use control room functions like DIM.
Then all you need to do is to create a routing by clicking to an HW output and
moving the faders of
- HW inputs, top row, audio from your UCX II's inputs in near real time
- SW playbacks, middle row, audio from your PC via USB with the usual latency over USB
This you perform for e.g. the outputs in the control room (Main Out and Phones 1).
You can store those routings in snapshots.
For Windows and applications without ASIO support you need to create Windows compatible devices.
This you do in the MADIface driver settings (see on Windows desktop on the bottom right in the Windows notification field).
Create WDM devices for AN 1/2 and make this to a speaker device in the next TAB speaker.
This creates dynamically "on demands" sound devices in your Windows audio settings.
There you set it to be the default sound device for windows, now you can also play audio with e.g. Youtube.
This routing to Monitors and Phones you save best to two of the 8 snapshot positions on the right side.
The whole setup you can Save as Workspace.
All this is explained in the manual and in the very good RME tutorial videos on Youtube.
Further information you find in this sticky, also a step by step guide how to create a good 1st time setup.
https://forum.rme-audio.de/viewtopic.php?id=34394
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub14