See objects only when looking a object with a certain shader?

I want a sort of "lens" which let's you see objects which have a specific layer which the camera can't see otherwise when looking through said "lens". Basically, a shader when you look through it which changes the Culling Mask of the camera only for that space. If it's not clear what I mean (or there maybe is a simple way of achieving it I didn't think of) let me give an example: Imagine a tube you can look at from different angles. When looking in the holes you see a room which is bigger on the inside. From the outside you can't see it because it's on a different layer, but at the holes of the tube are said "lenses". The closest thing I found to my problem was the first answer of this: https://stackoverflow.com/questions/52411588/how-to-programmatically-allow-unity-shader-to-control-which-object-renders-in-fr?rq=1

Any help appreciated