I ended up pushing it in until it stuck enough that it stayed there until something touched it, then I screwed its cap on over it to hold it into place. It is definitely not flush in the socket, but I think it has the connection it needs and is held there by the metal plate; I can’t see any differences in the operation of my phone.
How important is this antenna cable? I’ve spent 2 hours trying to get it back in and I’m pretty much ready to give up on it. There was no adhesive on my cable.