Use A Portal to Move Between Rooms with Impossible Geometry

I’ve been working on an FPS horror game. The character is in an infinitely large room with no lights except a flashlight. The door leading outside is locked, and walking around it reveals that there’s nothing on the other side. However, I want the ending to involve opening the door and seamlessly entering another large area. This involves impossible geometry. I thought the best way to do it would be using a portal-like system, where the user “Enters” the door and is teleported to the actual room at some other location. The inside of the door would have a view of the other room, and a player walking into it would appear to walk through it, like in Portal.
I have no idea how to begin creating this effect, though.
I’d also like to not have to spend $25 or $30 on a portal kit from the assets store for just this one thing.

I dont quite understand but i guess it must be either of the following,

If you want him to be transported to another location do this:

  1. Create empty game object with colliders and put it on the doors and name it portal
  2. Create a spawn point which is again an empty game object, in each room.
  3. When the player collides with the portal in room 1 , change the player’s transform position to spawn point in room 2 and so on.

If you want him to walk through the door,

  1. Remove rigid body and colliders( or use trigger)