Here’s how I got it to work in Cubase Pro 10.5:
At first, the device “Steinberg SKI Remote” was NOT present in Cubase 10.5. It should appear almost at the top of the list, on the 3rd position.
I quit 10.5, launched 10.0 which I still had on my system, and checked it there. Surely, it was present in the pull-down list and I could add it, so I did. Then I connected my iPad with the Cubase IC Pro app, played around a bit, and decided to launch Cubase Pro 10.5 simultaneously. I didn’t even know that was possible, but it works 
When checking the Devices list again in 10.5, the “Steinberg SKI Remote” suddenly appeared in the list, and I could add it there as well.
I then closed Cubase Pro 10.0, which made the app disconnect, and re-connected to what appeared to be the same connection in the app, but this time, it connected to Cubase Pro 10.5.
I hope this helps someone who has the same problems as me.