The DP cable works fine with other monitors on the same computer.

This is day 12 of using comrades as tech support.

Edit: I JUST FOUND OUT THE MONITOR HAS A SECOND DP PORT AND THIS ONE WORKS! @refurbishedrefurbisher@lemmy.sdf.org was probably right about the short. You know how new devices come with rubber plugs in their ports to keep dirt out? There was one plug in this monitor that I had never pulled out because I never needed it, and that was this port.

Now I'm on to the next problem with this new computer which is: why are xrandr commands not working and why are two of my monitors set at resolutions higher than their native ones?