Hello @Ciara__ 

The clients may still stick with router when  router's signal strength is not that weak.

Please locate range extender in the halfway between router and Wi-Fi dead zone and see if the signal become stronger and check if devices will switch to use extender's Wi-Fi (device shows up as Client on Tether app).  You may also set a different SSID for range extender, and manually connect some devices to the extender. 

( change Wireless settings via Tether APP) https://www.tp-link.com/support/faq/2541/