Not sure why you’re using Rhino for modelling such simple shapes.
Would be faster to do this completly in C4D, then to fix the import from Rhino.
As for your issue, i guess you should triangulate the mesh before the export and make sure to have a usefull UV map for it.
Here’s an example how the setup could look like:
As for the UVs i’ve created a secondary UV channel too for lightmaps.
The mesh has seperate materials for walls floor ceiling and the rest.
The UVs of the first channel are optimized for texture tiling.
So all you need to do is figure out the right values for U and V tiling.
(e.g. floor is 2:1 and walls are 6:1)
Also pay attention on how the collision mesh was set up, just in case you run into trouble with that on your own model.