@scousemouse 

Hello, may I have the model number of the TP-Link extender as well as the model of the new main router?

It is suggested to follow the below troubleshooting to narrow down the configuration issue:

What should I do if I fail to configure TP-Link Range Extender?

If the above troubleshooting doesn't help, please log into the web UI of this extender 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.