The Windows Device Manager must show the “Yamaha USB-Midi Driver (WDM)” below the “Audio, Video and Game Controller” node, if installed correctly.
Try http://www.midiox.com/ Midi OX, if it shows the Midi Port of the CC121 and activities of it. It’s old software but the 32bit version still runs on Win10 64bit. I see my CC121 at Midi OX incl. activities (Midi Port Activitiy or Monitor - Output windows of Midi OX).
Does Cubase show it at the Studio->Studio Settings->Midi Connections with status “active”?