iMac Pro 2017 USB-C ports not working

The USB-C ports on my iMac pro are not working - except when I plug in my iPhoneo r iPad.

Computer specs:

27" iMac Pro (2017), 3.2 GHz 8-Core Intel Xeon W, 64 GB 2666 MHz DDR4, Radeon Pro Vega 56 8 GB, OS 11.6

A couple of weeks ago, my external 5k LG monitor suddenly went dark and I lost access to my external HDs. I'm not aware of anything unusual happening at the time. The USB-A ports work but I cannot get the USB-C ports to work.

Assuming it was a hardware issue, I was about to take it into the Genius Bar when I tried one last thing: I plugged my iPad or iPhone in one of the USB-C ports and the iMac can see them. In any of the USB-C ports, they show up in the under Locations and I have full access to them.

Two weird things though: the devices plugged into the USB-C ports are getting power (the drive lights come on, but nothing beyond that) and WiFi has been acting up (stalling, then coming back, then stalling, etc.).

I have tried all the usual things: ran the hardware diagnostic (no errors), zapped the PRAM, reset the SMC, boot to safe mode, reinstalled 11.6, wiped the hard drive and did a clean 11.6 install. Nothing changes. I am at a loss.

Weirdly I saw that Other Word Computing was selling a used iMac Pro where all the USB-C ports and one USB-A port was not working. They don't have an explanation either.

Any thoughts anyone?

Thanks!