Do you use TM FX control room?

Then click to "Assign" Button and assign "Main Out" to the output where the ADI-2 Pro is connected to.

Then in submix mode click to the HW output "Main Out" in the control room and move the fader of all HW inputs and SW playbacks (top and middle row) to create the submix that you want.

If you only want to route audio from the PC to Main Out then use:
Options -> Reset Mix -> Straight playback with all to Main Out

Information how to use TotalMix FX with suggestions and links to RME video tutorials etc:
https://forum.rme-audio.de/viewtopic.php?id=34394

It would be good if you had provided the cabling, e.g., as an ASCII drawing because it makes everything better/quicker understandable. So if I understood correctly, your devices are connected like this:

clock
slave                                     AD           clock slave
UFX<-------ADAT1+2------------>16ch Summing Mixer---2ch->EQ---->Mix Bus Processor
  ^ AES                                  DA                                                               | AN1/2 OUT  up to +19 dBu, so that
  |                                       AD*)   clock master                                        |                    *) A/D conv. at ADI-2 FS
  +----------------SPDIF-------------->ADI-2 FS-----AN1/2 IN<-----------------+                    works without distortion
                        (coax.)        DA               \              /
                                                                \        [ANA] analog bypass
                                                                  \          \
                                                                   +---AN1/2 OUT-------------->act. monitors +  phones

[ Drawing as JPEG: https://www.dropbox.com/scl/fi/e0uhy1ac … 8&dl=1 ]

Suggestions:
1. You could use the ADI-2 FS with its high precision Femto second clock as clock master.
Then it uses its FS clock for A/D and D/A conversions in your setup:
a)  A/D conversion from Mix bus processor towards UFX via AES
b)  D/A conversion from UFX happens at the ADI-2 FS towards active monitors.
Side note: I am not 100% sure, whether you benefit more from the ADI-2 FS as clock master or if you made the summing mixer to the clock master. Then he could use his internal clock for the 1st D/A conversion in the signal path. All other RME devices are perfect clock slaves by SteadyClock (UFX) and SteadyClock FS (ADI-2 FS) technology.

2. You can use analog loop through, see manual chapter 9 (ANA): Then the analog input signal (up to +22 dBu) will bypass the AD/DA conversion. If you do not exceed +19 dBu at the analog input, then you can also use the A/D conversion (towards UFX) which is still useable.

Manual reference:
DE: https://www.rme-audio.de/downloads/adi2fs_d.pdf
EN: https://www.rme-audio.de/downloads/adi2fs_e.pdf

From manual (ch 9):
"The new option ANA implements an analog bypass from the analog input to the analog output.
For example, if ANALOG INPUT - LEVEL is set to +4 dBu and ANALOG OUTPUT - LEVEL is set to +19 dBu,
the ADI-2 FS amplifies the analog input signal by 15 dB.
At the same time, it sends it out electronically separated to three different outputs.
Since in this mode the analog output is no longer connected to the DAC, the entire analog head-room is available,
with a maximum output level of +22 dBu (balanced).
Even the input can now be fed undistorted with up to +22 dBu.
ANALOG INPUT set to +19 dBu and ANALOG OUTPUT set to +4 dBu attenuate such a signal by 15 dB.
Additionally the AD-converter is also active and usable.
As long as one does not exceed +19 dBu input level, the analog signal can also be picked up digitally and undistorted."