The Hub v3 connects to your existing network via Ethernet or Wifi but it does not broadcast Wifi and wouldn’t extend your network. I’m not a big fan of using hotspots for hub connectivity but it certainly isn’t unheard of. Unless I am missing something you should be able to connect a Hub v3 to the jetpack’s Wifi network and the Z-Wave devices to the hub. It’d result in a map something like:
cell tower <-> jetpack <-> ST Hub v3 <-> Z-Wave devices