I returned from vacation and couldn’t get the HDMI working on my UD-ULTC4K. The DisplayPort worked fine, as did the ethernet, keyboard, and power. But the HDMI wouldn’t work. I tried re-installing the DisplayLink drivers, etc. - nothing worked.
Then I realized that the USB-C cable I was using wasn’t the one that came with the UD-ULTC4K. I switched to that cable, and now everything works as before.
I don’t know the exact specs of what kind of cable is needed, but I do know that all of the other USB-C cables I had - even the one that came with my Macbook Pro - wouldn’t work.
Perhaps this will help someone else.