I currently have 4 HE hubs, one development C-4 and 3 production C-7's. I try / plan to funnel most of my notifications and geofence detection through one of my C-7 hubs. But I often get caught out by leaving the mobile app on my phone connected to a different hub (identified in the Settings tab of the mobile app) after looking something up or trying to diagnose a problem.
I think it would be nice to have the hub selected in the settings tab represent the hub used for geofencing and/or receiving notifications. From the tools tab, when selecting to connect to a hub, the user could be prompted to select their hub and mobile device. This way the "main hub" setting could remain in place, and the one-off connection can be chosen for that single connection and not stored in settings.
If you want to get a little smarter, if there is only one hub registered, and the user chooses the Connect to Hub option from the Tools tab, then perhaps you could take the Hub / mobile device details from the Settings tab, i.e. the current behaviour.