Are you able to connect you Roon Rock machine to a TV via HDMI. You should get white text, top left corner. If so, a photo of this could highlight an issue.
If not, could be a number of things. Whilst connected via HDMI, power off the Rook Rock machine, power back on, do you get the manufacturers splash screen, normally a logo.
If you’ve tried another network cable and port on your switch or router, then it seems likely that the ethernet port on your NUC is at fault. You could try a USB to ethernet port dongle as a replacement for the faulty port.