Thank you for the description of the issue as well as details about the setup and its cables. Unfortunately, the DisplayPort to HDMI cable is the culprit here…
DisplayPort to HDMI cables are not bidirectional, which means that they only function in one direction – from a DisplayPort output to a HDMI input. It will not work when connected backwards, such as from the HDMI output on the UD-3900 to a DisplayPort input on your monitor.
What other video input ports are available on your DisplayPort monitor? We’ll want to investigate other cabling options for you.