Blender City Problem

Hello Everyone.
i am new to UE4
I am facing this issue.
I generated a city using osm plugin in blender and exported in obj but when i imported it to UE4 everything works fine but when i zoom out a little bit City model turns in black and when i got close to city then i can see it again. Anyone knows hot to fix this?
Here is the video

If it’s all a single mesh it may be getting culled when you get a certain distance away, meaning it is hiding it because it thinks it’s very far away.

Yes its a single mesh.
Should i Import everything separately?