After you add them to smartthings, unplug the hub and leave it off power for about 15 minutes. While leaving all your other devices on power. Then when you power it up again all the Zigbee devices will look for the best route, which will cause them to use the new repeaters.
Also, you might find the following helpful:
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) you need one device that can repeat about every 40 feet…