OK. Problem solved. I did a BIOS restore factory settings and the ports came alive. I will leave the thread up in case it is of some use to another Optiplex owner.
The 2 USB C ports on my Optiplex Ultra 7090 are not active (but do provide power to devices). BIOS settings are OK (all USB ports are set to "active") but by the time it gets to Windows they are effectively dead. All drivers and firmware is up to date and the PC is virtually new and the ports physically look fine. The other USB ports work fine. Any suggestions?
USB C Ports not Recognized by Windows 10 on Optiplex Ultra 7090