@Matt1888 

Hello, is the extender model RE200 or RE220?

If you cannot connect it to the new Sky router, please try below troubleshooting:

Please follow the instruction to log into the web page and tell me the current hardware and firmware version.
https://www.tp-link.com/en/support/faq/1398/

If the extender firmware is not the latest one, please update it first.

https://www.tp-link.com/support/faq/1406/

And then please reset and reconfigure it and check whether it works. 
https://www.tp-link.com/en/support/faq/1400/
Note: You can change the authentication type and encryption type of main router's wifi as AUTO.

If there is still only power light on, please relogin and show us the full screenshot of the status page. And please export the system log to us which you can find on the settings-system tools-system log and please also confirm there is no special setting on the Sky router such as access control or MAC filtering.

Thanks in advance, and have a great day.