Translation:I'm outside the room.
Maybe a native Chinese speaker can tell if I'm right or not, but when I here "我在房间外“, I have the impression that the person is just outside the room (in the next room, down the hall etc.) But your English "out of the room" is very broad; the speaker could be anywhere except in the room (from the next room all the way to the other side of the planet). But if this impression is wrong then it should also be accepted.
I'm not native Chinese either, but I think your impression is on the right track. Technically, I think the Chinese sentence could be taken to mean anywhere outside the room, but I don't think anyone saying it would have that broad of a meaning in mind. So I think "outside of the room" is technically correct, but probably not the best answer. If someone wanted to give that broad of a statement, they would probably just say 我不在房间里.