Pic 1: It seems your normal of the top plane is reversed. You can ensure your normals are facing the correct direction in Blender for exporting. For a quick check, you can try checking “Two Sided” inside of your material properties which will ignore culling the backfaces. I would fix the reversed normal though as it would be the correct fix.
Looking at the shading, I don’t think there is a top plane. @Baziga, is the top model meant to be a hollow tube? If so, look up “Back-face culling”. If it’s hollow, you do need to see both sides so open up the Material you’re using, and turn on the “Material → Two Sided” checkbox in the Details window.