I’m also having problems joining another Z-Wave network. I noticed that the Android mobile app doesn’t even have the “Join/leave another Z-Wave network” option, which is suspicious.
When I use the web app to select this option nothing happens when I try to add the hub as a secondary controller to my primary controller. I know that the hub Z-Wave functionality is working because I can add a new Z-Wave dimmer and control the lamp with the mobile app.
It seems like Smart Things isn’t supporting the hub as a secondary controller. I’ve opened a support ticket about this issue.