@munro-pt 

May I know the model of your Wi-Fi router?

You could to forget router's Wi-Fi on the Mobile phone and reconnect it ,this is to make sure Wi-Fi password still works on other devices. 

• Go to Settings→ Wi-Fi on your phone, Tap (or Long press  the router's Wi-Fi name  →Forget / Delete the network 

Please reconnect to your router Wi-Fi it will ask for a password, will it accept your router password?

• Later  factory reset the range extender go Setup through Web page. 

RESET: With the extender powered on, use a pin to press the RESET button until most LEDs flash once. 

If very few cases, router's password changed in WPS process,and to tell if router's password becomes a random long wifi password, you could download Tether app and check the Range extender's password :  Why is my main router’s wireless password changed after using WPS to setup the Range Extender?

Hello @munro-pt 

Thank you for your detailed information, your case seems to be related to WPS process mentioned in FAQ I posted above.

After factory reset, range extender's default SSID will become an open network again which does not require a Wi-Fi password, so you could connect to extender's WiFi then later login its Web page. Or you could try to set it up through Tether app.