Cull set to off disables culling, producing a double-sided material, however light is still calculated according to the normal of the mesh's face and, as result, it will not render correctly for the back face, which is the expected behavior.
There are a few ways to work out this situation, the simplest one by using our Switch by Face node to provide different inputs for when each face is facing the camera.
Please let us know if you have any further questions, thanks!