I’m very new to modelling but have been creating very basic objects/buildings. I’m having a problem when importing a prison cell, everything imports fine but when i go inside the cell the walls are invisible, the outside faces are visible but when inside they disappear. Help would be appreciated 
That’s backface culling. You will have to make your mesh “solid” -> you can do that in your 3d program (e.g in blender there is a solidify modifier).
Otherwise enable “two sided” in your material
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