The UniFi Dream Machine Pro has little rubber feet. If you want to pass cables underneath then you need to raise the DMP up, or buy a rack and patch panel.  I choose to make some feet. 

These use the mounting screw holes at the front to secure the front feet. The rear feet just sit there. Use the original screws and you'll get a decent fit.  I think the holes could be a few tenths out as I measured rather than looking up the exact fitment spec!

You will need to remove the little rubber feet at the front for this mod.