@Alfie79
Look at the LED light on the extender and see if the LED signal light indicates it is connected to the router. If it is then the extender is likely broadcasting the same network name of the router. It is not that it is wrong, but just that is the case.
If you want to change the extenders Wi-Fi name you need to log into it either using the Tether (https://www.tp-link.com/us/support/faq/2540/) app or through a web browser. Find the IP of the extender by looking at the router connected devices, then enter whatever the IP of the extender is in the browser to log into it.