Models get disfigured when imported from Blender to Unity

When I import my Blender model into Unity some of its outer faces are "transparent from one angle and if i rotate around it becomes visible but then the other side becomes transparent. Also you can see the inside of some parts. My current Unity version is 2019.3.5 and Blender version is 2.80. I’ve tried to fix with flipping the normals but it doesn’t help.

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