The trim ring, which is fastened from the back (inside the wall), has a rabbeted edge which lips over the inner edge of the door frame. To work properly, it has to go all the way around the edge of the frame. It isn't a flat trim, but has a cross-section which looks like this. In the picture, the top part rests against the wall (on the inside face) & the lower part clamps against the door frame edge with the screws.
The screws go through the back side of the trim ring, then self-tap into a "lip" which is around the edge of the door frame. When it pulls up properly, the trim ring is fastened to the metal door frame itself & acts like a clamp, which pulls the gasket tight against the outside skin by pulling from the inside (back side) of the door frame & wall. The inner side of the wall (or the inside of the cabin wall) is what the trim ring pulls against.
Here's a picture of what it does on the inside of the cabin. The left side is in place where it would go but the right side is "floating" due to the fit.
With the metal clamp piece in place to hold the trim ring together, there was no way to make the trim ring fit around the door frame. It would fit around three corners, but it was too small to fit around the 4th corner, no way to make it fit over the back of the door! That's what the first pictures show in the above images. The trim ring is too small to fit around the door. Hope this makes more sense now! I'm getting confused just trying to write it down again. If I get the #*^^# door installed, I'll take a picture!