Object with millions of polygons

Hello guys, I’m having a big problem when exporting geometry with millions of polygons to UE. The 3d model was done by 3d scanning a physical model and then converted the point cloud to meshes. The result is a mesh with more than 6 millions of polygons. I could simplifly, unwrap and export the fbx. When I import it in UE I get the model and it looks totally fine until I build the lights. I’m using 4.12.5.

This is result after importing and adding a single texture:

And this is what happens when I build the lighting even with the highest quality:

Anybody has a clue of what I am doing wrong? or knows any other way I could get this model in UE?
Original mesh:


PD: I don’t know why it gives me error every time I try to insert the pictures.

I think something to do with normal flipped…unify all the normals or check the unwrap UV have any flipped normal.

It would be better if you retopologized the geometry.