Render only part of 3D mesh that is on 2D window

Hey I have a problem with rendering an object "inside" a window. I've tried multiple things with shaders but I'm not very good at writing or editing them so have not found the solution. See the picture below to see what I mean.

Unfortunately the objects cannot be changed to either all 2D or all 3D thats why I'm looking for this specific solution. Masking works when I apply it to the 2D object but that doesn't fix my problem.