How to Fix invisible Faces in Unity
Show missing parts and faces of an imported (through Editor or Runtime) model in Unity by duplicating the faces of the imported meshes and reversing the faces' winding order and normals.
Unity Answer on How to make visible the back face of a Mesh By Aldo Naletto:
https://answers.unity.com/questions/2...
The issue of missing faces in Unity could also be solved in the Shader.