In the game’s current state, there is absolutely NO way for a disconnected player to get back into FotM with his party. In fact, if the leader of the dungeon tries to re-enter after a disconnect, the entire party gets kicked out.
This is the only dungeon that does not allow players to re-enter after a disconnect and it’s extremely frustrating. At the very least, allow original party members to re-enter the dungeon if they disconnect, just like every other dungeon in the game.