Unless you’re experiencing some sort of glitch, it’s fairly easy to configure a door. Click on the door, then click on the first drop down menu. You can’t connect it to another door unless you first choose the level it’s going to (you can also choose the level that the door is on). Then, use the second drop down menu to choose which door it’s going to. (Note that you can use a collision or non-collision invisible actor to teleport your hero to any door, but you can’t use a door to take the hero back to the invisible actor.)