Situation You need to open a door with a key on a Shabbat belt in a place with no eruv.
Status You may not disconnect the key from the belt, stick the key in the door, and open the door in a way that the key enters a private domain.
What to Do You must either:

• Turn the key while the key is still on the Shabbat belt, OR
• Remove the key from the Shabbat belt (or take off the belt), open the lock, and then replace the key on the Shabbat belt before you open the door.