Hello  @GregoryThomas 

You could check the model & hardware version of range extender and make sure it is in the Tether app compatiblity list,

Tether Compatibility List

• To let Tether app detects your device, confirm the mobile device installed Tether APP and the Range extender are in the same local network, 

turn off Mobile data and VPN on the phone, connect phone to Range Extender or router's Wifi network and open Tether app again

If you'd like to reinstall the range extender, try a factory reset and reinstall extender as guide here. Configuration Guide TP-Link Range Extender Setup (Range Extender Mode)