The invisible walls might be caused by wrong face orientation. Have you checked it in Blender?
I am not sure what u mean as in blender it all looks the same
If you use planes as your walls they have front and back face (not sure about the proper name in english) and in Blender in the upper right corner in viewport overlays you can toggle on face orientation. Blue colour means front (has material and is visible in blender) and red means back and it might disapear in UE.
I would recommend using walls with thickness so that you can see them from all sides.
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