I need help with my grass material!

During the last couple of days, I tried to setup my grass material with all sorts of methods I found online.
I ended up using the two sided Foliage material, and I set it up like this: https://wiki.unrealengine.com/Two-Si…liage_Material

My material

But the shading is a mess…The back face of each plane is very dark.

I tryied to use the node TwoSidedSign multiplied by the normal map, same ugly shading. Unticking Tangent space normals give nice results until the directional light changes direction.

In the mesh viewer, it shades nicely, like so. And, as you can see, vertex normals are pointing upwards.
What I’m doing wrong ?
Btw, does it need to have a normal map plugged in ? I try with and without normal map, same results.