Object with more than 255 polygons (with Mesh Collider(Convex - on)) is falling through a Terrain

If you're moving object can't be a convex meshcollider (due to more than 255 polygons) you can't use a MeshCollider at all. A non-convex MeshCollider can't collider with other non-convex MeshColliders. The terrain collider can be seen as non-convex MeshCollider. So your only option is to either use a custom mesh that fits the original mesh best and can be convex, or use multiply primitive colliders as compound collider to get an approximate collider. The second option is probably the fastest one.