Sniper Scope lens problem with Render Texture

Hi guys i got a bit of an issue with my scopes for my FPS Game, i know the theory behind it and how to do it but the lens itself is making me go crazy. as you can see in the image, i have 2 GameOBjects. 1 is a circle i made in sketchup, i also tried the same thing from blender, but i decided on making it in sketchup exporting as dae and re exporting it through blender as an FBX to use in unity. but the biggest issue is that i can not get a render texture to work on anything that is not a gameobject default of unity. could someone please explain what i am doing wrong here. or is the render texture simply only available to unity default gameobjects e.g Cube, Cylinder(like i am using in the second part of the image), plane. thank you for your help on this subject.