Have you tried plugging the top cable into a separate desktop monitor, to see if it displays correctly?
If a good is displayed (only size may be off), then the cable, and video card would be okay.
If you get to swapping this far, then you'd want to try plugging a desktop computer into the top monitor to see if the problem is the monitor.
If that doesn't work, then it could be the power supply having a problem because you already know the cable is good.
Last thing to try is swapping the power cables between the topbox and main screen...I believe they're the same but one is longer than the other?
Just giving ideas on swapping stuff to get to the root of the problem.
There's nothing really special about the top monitor....it's just an LCD computer screen like a desktop monitor.
Read this link I've attached to here....the fella had ALL KINDS of cable problems....>>>
https://newlifegames.com/nlg/index.php?topic=38313.0