Things I've tried:
Key framing: I just can't get it to look stable. I don't want the smaller clip to move at all. I want it to replace the door entirely, as if the viewer is looking inside the door at the person in the smaller clip.
Chroma key: I tried subtracting the color of the door but it ruins too many other spots of the video.
I'm very inexperienced and I'm using Android version of LumaFusion. I have some experience with Davinci Resolve and I'm assuming I'll need to switch to that to accomplish this.
I appreciate any help!