With 3D models, each face has two sides, the "normal" face and the back face. In most setups, back faces are never rendered for performance reasons. That's why "normal" faces exist, to tell the rendering engine what to show on the screen.

In Blender, for example, the "show normals" button will mark with blue lines which face is the normal. Unity is not showing the back face because most engines don't. Since you don't have a normal face there, it's see through. To fix this, you can try this:

If you need that face to be transparent, make a face there, don't cut it out as you have, and then make sure the material for that face has an alpha channel that is see through. That way you have a normal face but can see though it.

To actually see the face inside though, you need fake a "double sided" face by making a perpendicular face to the back face you want to see. Then texture it as normal, and you should be able to get the desired effect in Unity.

I know it's not a simple solution, but it's a quick and dirty way to see back faces easily.

Back sides of a mesh are normally Culled. You can make them visible by using a different shader with "Cull Off" - google for that and you'll find heaps. Alternatively, duplicate your entire geometry in Blender and flip normals (there may be a filter to do that automatically).