confused about how to handle fbx and dds files

I'm trying to import a modification from assetto corsa.
I installed a folder that contains all the fbx and dds files needed to construct a landscape in Assets folder of the project.
And I tried extract textures and materials but it doesn't work.
The object can be seen on the Scene window, but remains gray overall. Apparently the color and surface information is not reflected.
I checked the materials. The dds file is attached to the material, but that information is not reflected in the material and the material remains gray.
How do I get the dds file to accompany the material correctly? Or how can the object be drawn with the texture of the dds file?