I have very high poly mesh behind a simple unity plane. FPS drops somewhere around 10fps when the high poly object is active even if its hidden behind the plane.
Its a new project
Camera Occlusion culling enabled
Dynamic Occlusion in meshes enabled
does occlusion culling only work if objects are behind the camera?
my understanding is I think camera culling is referred to as Frustum Culling, and yes, this only works the the bounding box of the object is outside of the camera view, e.g. behind it.
I think regular unity occlusion culling requires baking and does not work with dynamic objects, e.g. a plane being toggled on and off. You would need to have a static plane, bake it as an occluder then move the camera so that the plane occludes the view of the high poly model for culling to apply.
There are some real-time occlusion culling techniques available e.g. as assets on the asset store or you could maybe have a play with this: