Instructions - More or less.
Have your home wifi name (SSID) & pw ready.

Plug in the new router/extender somewhere you know is in range of your existing home wifi. On your phone or laptop click this link.
http://192.168.10.253 (you can also scan the QR code on the carton, same thing really).

You will be asked to login.
User name: admin
Password: admin
A web page from the extender should appear.

Click WiFi logo.

Enter the name/SSID of your house wifi & the password (key).
Leave encryption as is.
Press apply and watch for confirming message.
Done.

You should now connect with the same password as your home wifi has. Any device that has it stored already should log on automatically and connect to the base or the extender as needed.

If you unplug the router and plug it in again it should remember your settings.

Plug it somewhere close enough to your base router to be in range. High up is good. Block, brick or anything damp is bad. Damp people block wifi.