Digital video is confusing. You get some clear advantages, since signal degradation becomes a thing of the past, but if you’re not someone who works in video for a living, it’s difficult to keep it all straight. And standards are a problem. You can’t just assume that two devices will work together because they’re both “digital.”
One of the problems is physical incompatibility. Some devices have Displayport ports. Some of them have HDMI ports. The solution is easy: get a cable with an HDMI connector on one end and a Displayport connector on the other. Problem solved.
And now the guy who sold it to you is a criminal. (You aren’t necessarily. Possession isn’t illegal, just sale or manufacture. So don’t sell it at your garage sale in 2019.)