Go back to C4D, open the file, switch to poly mode, and select some of the floor polygons. If they highlight in blue that means the normals are facing the wrong way and you can reverse them in the structure menu. If they highlight in red/orange then those are fine and the problem may be n-gons in your model. In poly mode again, select all and remove n-gons from the structure menu and hopefully that'll solve it. Unity will automatically triangulate quads in your models but it doesn't know how to deal with n-gons, and that can make certain parts of the model become fully transparent in your project.