Hi Fynn, congratulations on your first licensed app!
Demo users only work in local and free apps, as mentioned in the project Security where you configure demo users.
In the Mendix portal you can set the MxAdmin password (Environments > Details > “Change Admin Password”). Make sure MxAdmin (only) has rights to create admin users. Login as MxAdmin, create an admin and then login as that admin to maintain the rest of your users / application.