Could it be that you try to run the Performer and Cubase on the same computer? That doesn’t work.
There must be 2 computers involved. One is the Performers’ computer (or iPad) and the other is the engineers’ computer running Cubase.
When not running on a local network, the engineer clicks the login button and gets a number (a key) in the text field. He tells the Performer that number and then the Performer types that number in his login field and clicks connect. It has to be exactly this number (with a blank (space) between the 2 groups of numbers). The message “Studio with key xxxx xxx not logged in” means exactly that and is only displayed in the Performer app.
When running on a local network (only works with the PRO version), the Performer should see a green LAN button next to the login field, provided Cubase with the activated VST Connect PRO version is up and running. Just click the green LAN button and select the displayed IP address to connect.
Does that help?