Help with importing a blender built building and adding textures

Hi,
I’ve built a medieval house in blender to import to unity in a fbx format for use in Unreal. I have two problems. One, that the back of the building doesn’t show when I’ve imported it, even though it’s there, but the main problem is adding textures. I’m very new to creating levels, more so with custom content. For some reason, Unreal is only letting me add two textures to the building (i’ll add a pic with colours to emphasise this), and only in certain areas. But there are places to which I want to paint differently to what it’s allowing.
Any thoughts on how I can change it so I can paint where I wish?