Some things:
- Using Z-Wave Plus only will boost the range of the mesh - once you add any non-Plus devices the range of the mesh will drop. I found a huge difference after I removed the last non-Plus device.
- The mesh is built by the (mains/USB) powered nodes only - any mains/USB Z-Wave Plus device (it doesn’t have to be a repeater) will do that
- Graphing your mesh will help you understand the connectivity