New Router - New Password

I have received a new Router from my ISP. I previously used the N300 WiFi Range Extender on the old Router. I have set up the new Router but can't seem to get the Extender to 'forget' the old one.

I have tried the WPS method of setting up but all I can see is the old Router Name with _EXT after it. It won't recognise the new Router and dispose of the old Router details and Password.

I have tried to reset to factory settings but it still remembers the old Router. It's getting frustrating!! Why can't I get an Extender with Alzheimers?

How do I get to start with a clean slate and set up the Extender with my new Router?

The brochure instructions don't cover this.

I am on Windows 10 and trying to extend the WiFi to the back of the house to be able to use my iPad and Smart Phone.