When the player opens a door, the door knows where the player should be after the level knows. How it does that is up to you, but if it’s a BP, you could have something in the BP which marks where the player should spawn when the next level loads.
So, in the door BP you can say:
Now that coordinate is in the game instance, so when the level loads, you can say:
Boom, player’s in the right place, next to the door they just opened.
EDIT: It can also be that the door doesn’t know where to put the player. In that case, the new level must know where each door is connected and have references to them, they could be numbered, or whatever. Then, the door just has to say WHICH spawn point to put the player at.
The idea is that somehow the door puts something in the game instance, which the newly loaded level can use to figure out where to put the player.