Polygons on model are missing, but looks completely fine in Blender

Hello. I’m making a VRChat world and I’m using a model a Tumblr mutual provided me. There’s only one problem: the models appears broken in Unity. Polygons that are part of the model appear to be missing. Strangely, these polygons are only invisible from one angle. For example, outside the model a wall appears missing. But if you go inside the room with the wall, the polygons making up the wall are no longer missing. The model looks completely normal in Blender, and I’ve already tried importing it as both a .blender file and a .fbx file. I’m not sure what to do anymore. Is this an exporting issue? How do I fix this?

by default blender shaders don’t cull the backface, they can if you want

backface culling is the term. usually stuff is designed with an inside and outside. like a cube, don’t need to draw the inside walls

the recent toon standard shader has a culling option, if you’re thinking about quest. otherwise poiyomi has culling options

So in order to fix the issue I need to change the shader? If so, how do I do that?