Zigbee only extends zigbee, Z wave only extends Z wave.
Start with this FAQ, and in particular, read post 11. It should help a lot.
One more thought. While I understand the marketing reasons for the decision, the fact that SmartThings gives customers zero guidance on how to set up a network backbone does lead to a lot of frustration that shouldn’t have to exist. if people knew that .1) The hub should be located centrally in the home, both vertically and horizontally. Don’t put it in a garage or basement as those have concrete and pipes and metal objects which will reduce signal. If your Internet router is off in some inaccessible corner, you can use a Wi-Fi access point that has a ethernet port on the side and plug into that. Also, the hub should not be put in a cabinet. You just want to make it as easy as possible for signal to spread around your house. .2) they need one device that can repeat about every 40 fee…
After reading that post, just scroll up and read the whole thread, there’s a lot of helpful information there.
And if you want even more information after that, see the article on repeaters in the community – created wiki.