Commander core XT is installed but not showing up in iCue, so I can't control the fans. However my Corsair H100I Elite AIO is plugged into the Commander core and I see it just fine. BIOS is up to date. ICue is up to date. I'm Running an Intel 7 12700 and a ROG STRIX Z690A D4 Motherboard. Literally everything else is Corsair. PSU, Fans, Case etc.
So if that's not connected that could be an issue. Also, make sure that the USB port is enabled or try another port. Sharing some more system info would be helpful too. Is it possible for corsair to send me a new one I've tried unplugging it and plugging it back in and it still won't work properly.
It should work. You could do a full power down of the system and try to boot it back up. First time I booted my CoPro with a H100i plugged in, it wasn't detected. Try swapping headers between the capellix and the commander pro. Note that it is easy to plug devices offset by one pin on the commander pro USB header..