The Korg drivers can be a problem. They will show up just fine in Device Manager but will not actually be viable MIDI ports. Not just Cubase, even the Korg Control Editor won’t see it. It has something to do with an old limit on the number of allowed midi devices - don’t recall the specifics anymore. But I’ve had to fix this twice and it took editing the Registry and removing/re-installing midi devices. You’ll want to download the utility - Uninstall Korg USB-MIDI device.
I found fixing this pretty frustrating as making the same/similar changes won’t work until suddenly it does and it’s not clear what you did differently.
In general you should be able to have multiple USB-MIDI Devices available at the same time - I have 6 different ones (now) running on my DAW - from a variety of vendors Steinberg, Behringer, Line 6, NI and most of the time but not always Korg.
I did a search for KORG on my browser history and here are some links I found related to the issue. Good luck.
