I'm not sure what is going wrong here. Sierra should work okay (lots of users are using the Z-Stick + Sierra, and we've tested it here as well), although I think there might be some USB problems or incompatibilities with older Macs that have been upgraded to Sierra. I'm surprised that when it is in the USB hub it doesn't show at all. That makes me wonder if the Z-Stick is malfunctioning, especially if it was working well previously but no longer is.
On the other hand, if nearby devices are working but ones beyond 30 feet or so aren't, then I think it is a RF or range problem. Try unplugging all RF electronics you have to see if that helps. When you do the network optimize, are the modules you've put between the Z-Stick and the unreachable modules finding the modules that aren't reachable? The Event Log should show you the details.