S/PDIF1/2 can only be connected to Main In5/6 (as an alternative to AES1/2). But, S/PDIF3/4 can be connected to any Main Input (as an alternative to AES11/12).

Is your problem that you cannot connect SPDIF3/4, or that you can connect it and it does not work ?

In either case what do you hear, and see on the screen and meters ?

This stuff is complicated, because there are so many options, and a number of restrictions which make it harder to understand.

You should be able to get an output with no dry (direct) signal. This will depend on which routing and preset you are using, and also how your external equipment is connected.

First try loading a suitable Factory Routing ("Analog A-B") and Preset 11 "Mute". You should be able to adjust Wet/Dry to give no signal at 100% Wet and full signal at 0% Wet. If this is not so, the problem is in your external equipment. Otherwise, then try loading a simple pitchshift effect such as 313 "4 Pitchshifters". You should then be able to select between 100% Wet and 100% Dry. Let me know.