I have had this Omen 870-200 for a few months now and just got around to adding a second monitor.
My primary monitor is using the single HDMI port and I am trying to use one of the DisplayPorts for the second monitor but the computer is not recoginizing the monitor and the monitors will go into power save due to no connection. I'm sure the all the monitors work via the HDMI so it appears that the 3 display ports have some issue.
I have update Nvidia drivers. Still not working.
I have 2 monitors that I could possibly use.
1 monitor, the computer will detect but the monitor but only in 1 of the 3 displayports and will shut off due to no signal (and I've now managed to assign that monitor as the main so if I plug it in my true main monitor shuts off)
There should be nothing special to do to get a monitor to be recognized in any of the ports as long as they are the physical ports on the Video card itself and not the other ports on the motherboard. The motherboard ports are disabled by default.
IF you are having issues with the ports on the physical video card, you will need to contact HP Support directly for warranty issues.