2) twist the spring handle to loosen the damper,
3) pull out the burn basket assembly (maybe vacuum any ash while you’re in there)
4) through the now empty burn chamber, see if the damper door can be reached with something long (a large dowel, a crowbar, etc)
5) put that "something long” thing against the closest edge of the damper door and give the thing a whack or two with a hammer.
With any luck, beating on the damper door through the burn chamber will knock it loose. My stove is fired up at the moment, so I can’t check to see if this actually possible. I hope it is and this works for you.
Good luck!