Occlusion in Augmented reality for portal

Hello, I am using AR template from unreal engine to place a 3D model on the floor. My question is, how will I achieve the effect they do in the video below where everything is hidden except the entrance to the portal and then you can see all your 3d model once you are inside.
I tried using this tutorial but not sure if its the right direction?

Hey, bro, are you finally finish this function?