I’m trying to make a similar thing like we have in “Hellblade” game.
How to render (or hide rendering) some object only through a portal?
Okay I solved it, looks easy 
Apply this material to the object you want to be visible through a portal (or remove “1-x” for reversed case).
Portal is just a mesh (plane) with default material and disabled “rendering in main/depth pass” + enabled “render customDepth”
One more way with opacity mask usage - Unreal Engine 4 Tutorial: Antichamber 4D Art Gallery(Impossible Objects) - YouTube