Planes, having no depth, are almost always considered to be one sided, as they aren’t really a solid shape.
For what you are doing, you will likely be better off with a thin cube (giving you 6 effective planes on one object), as planes rarely look right unless backed by other geometry.
If you want to stick with planes for whatever reason, Go into the material that is on your plane and click the ‘Two Sided’ option, which will tell the renderer that you’d like this plane to render both sides.
Hope this helps!