Hello, 

We have a configuration guide that helps to configure the range extender step by step. I believe it will be very helpful for you. 

[Configuration Guide]  TP-Link Range Extender Setup (Range Extender Mode)

RE Setup Video: https://www.tp-link.com/support/download/tl-wa850re/

It contains 3 methods to set up the extender, you could try Tether app at first. After following the video, if you fail to set up the extender, please feel free to contact us and provide us which method you have tried and which step you get stuck with.