Unity does not support run-time mesh import from files outside of the project for performance. Fortunately, AssimpNet provides nice Unity plugin enables loading various formats of mesh objects.
I wrote a Unity package based on this: UnityMeshImporter. This package loads mesh file by AssimpNet and convert it to a Unity GameObject. The source code is available on GitHub.
It just works like this:
You can find a simple example Unity app here. Please star the projects if you like it. Any comments or issue rerporting through GitHub issue is welcomed. This project is also used for RaiSimUity project.