ISM Culling in Blueprints in 5.1 why aren't they working?

I have made a copy of my 4.19 project and opened it in 5.1 I have converted all meshes to Nanite and now when I FreezeRendering the Frustrum occlusion culling is not working the same as in 4.19. I have a large procedural building and I had a good frame rate in 4.19 but now in 5.1 it’s way down. Why is this and how can I fix it?