Static mesh imported from 3ds max is very dark, even tho there is an lightmap (Unwrap UVW)


I have an static mesh for the floor of the hall that I’ve prepared in 3ds max. It’s just the concept for now, so it’s very low-poly.
Before exporting it to FBX, I’ve added the Unwrap UVW:

Now, inside Unreal Engine I’ve imported it and made sure that Light Map Resolution is 512 and Light Map Coordinate Index is 1.
But when I build lights, the floor looks way too dark. I’ve tried it on several basic materials with exactly the same result.
For comparison, I’ve placed an UE4 Plane with the same material next to it (on the right side of screen) - you can notice it’s way brighter:


What am I missing here?

Exporting without Unwrap UVW gives me totally black mesh (as predicted), but adding it gives only “a little” light on it. Still, as you see above, UE4 reads the UV channel (preview in bottom left).

My FBX export options: