Hi Frieder,thanks for your response!
And sorry,i forgot to add something important:i guess what you experience is that many(most?)synths apps have all available Midi inputs"on"by default.So you are not playing them through Cubasis,they will always play no matter what channel or midi output you select in Cubasis.Try it with sunrizer,go to the midi settings and you will see all available inputs are active (Nanokey,Cubasis and Seesion 1 in my case).Deactivate your keyboard input and try it again.You should not be able to hear a sound then.Although it's still possible to use cubasis internal keys and sunrizer will respond to the recorded midi notes (that you recorded with your external keyboard!)as well.
A small workaround could be to record your desired notes (from sunrizer etc.)and change the midi channel in cubasis and sunrizer to a different one.The Nanokey sends midi by default on channel 1 only (same like most other keyboards i guess).But it wouldn't be the most comfortable solution