Hello
I have a problem and tried all the solutions I to no avail
When making house
I do export to the engine
The problem here
From the outside there is no problem
But inside the walls disappear
://i.imgur/hRozqNY.jpg
ins
://i.imgur/GnwvNwi.jpg
and
://i.imgur/PuP7buu.jpg
://i.imgur/6NIWIou.jpg
You have to make your walls “solid” (e.g in blender you can add a solidify modifier) -> because now you dont have a backface. You can also enable “two sided” in your material 
generally better to build with solid shapes that have at least a front and back face rather than just flat planes.
build things with some thickness for best results. if you make double sided polys they will still be infinitely thin and that has a bunch of problems that tend to creep up on you. Ok for sprites off in the distance, but build shapes with volume for anything you get near.
As the others have said; you’re using flat planes. Use solid walls instead. And make them (and all of your models) 1:1 scale with the real-world; just in case you want to implement VR at some point.