I just had same issue trying to upgrade TIU to 6.10. No serial port detected on the DCS 5.0 loader.
I had used the same USB to serial cable for the upgrade to 4.2, but it was on a different laptop.
I went to the maker of the cable (sabrent.com) and downloaded the driver for that cable. Windows 10 Home Edition.
Eureka. Was able to upgrade TIU and remote.