This article explains why video might not be working, although connected via a DisplayPort cable. Whenever pressing the OSD menu button the menu only appears for 1 or 2 seconds and then disappears and possible solutions

If you are connecting a DisplayPort video input to an Elo digital signage monitor but are not getting anything to display on the screen, you may need to change two display settings in the OSD menu of the monitor, please see below for details and how to fix.

On Elo's IDS monitors the default video source is HDMI. If the Auto Scan and Port Discovery features are turned off, the video will always default to HDMI and will not pick up the DP connection. To correct this you will need to go into the Video Settings section of the OSD menu and turn ON these features.
 
NOTE: You may need to connect an HDMI source to the monitor to be able to go into the OSD menu. Once in the menu, make sure both "Auto Scan" and "Video Port Discovery" are set to  ON. After turning on, disconnect the HDMI connection and connect the DP cable. The monitor should now auto scan and discover it the video source.