Well i’m sure you solved it by now.
But i saw, in your file, that you have 2 models on top of each other.
I deleted the mesh version and kept the OpenSurface version.
I moved all close to the origin(it’s always a good practice to do this, regardless of this particular problem)
When i tried to export the model to DWG, it worked but it was missing one part of the model.
Obviously there was a problem with the geometry, not clean enough, so i exploded the whole model(was too lazy to check the topology!)
Now it’s made of small chunks, then i exported it to AutoCad. Everything came clean.
Here is the AutoCad File: https://drive.google.com/file/d/1AP4j8sXxxDrs0cB6R1b-X0DLL26GYxCH/view?usp=sharing