Lighting Problem, please help!!

I’ve been working on a simple room in UE4. I started out making the actual structure out of UE4 cubes (Scaled), but found that at the seams at the corners (Floor-wall, wall-ceiling, etc.) there were always dark shadows. So I decided to model the room in Blender. I’ve made sure the normals are all correct and created a 2nd UV channel for the lightmap, and smart UV unwrapped it with pleanty of margin (See image).
But now, in UE4 there are still shadows in the corners and half the model has gone black… I’ve made sure all the faces are on the lightmap, unticked generate lightmap UVs and put the lightmap resolution up to 256…
Any ideas?