That sounds like a small entrance like a door, best with a door your can make open and close for have good transition.
I have a third-person game and buildings with bigger entrance, like big garage (or hangar, or other buildings with very big entrance, or big cave entrance). It’s open, you can look inside through the big entrance and you can look far inside.
I do not see a solution here with any kind of “transition”. How should I do the transition when the problem is that you see the skylight in the buildings already when you are 20 meters away from the garage and look inside, which already is the problem. Of course I could shut down skylight when “going” inside. But there is no really “going” inside transition". Yes there is, but the transition is not on point, the transition is a long way and having mind, the TPS character can turn around and should not see how “streets” get darker when go into the building.
Of course I love simple solution and often game development is trying to make things simple, but in this case I think is is not working. Or I miss a point in the solution. Or I am simply dumb as hell. 
I heard / read about solutions using a transparent darker wall (I think you mean that with “entrance portal”), like dark glass, which makes the look inside darker. But I struggle to think about this solution in my third-person example with big entrance where you can look far inside (and also look far from inside to outside when turn around on way inside). When do I should use the transition and remove the “dark glass walls” and switch off skylight and having it smooth (when player turned arround)?
If someone knows a tutorials or UE4 inside stream or somethings else where this topic is explained, would be cool to have a link.