@Max72 

Hello,

It is recommended to use "Tether app" to change Wireless settings for range extender at first: 

Here is a link you can check if your product support Tether: https://www.tp-link.com/en/tether_compatibility_list/

Please according to Additional step 2 to customize the wireless settings: https://www.tp-link.com/en/support/faq/1373/

Or you could change Wireless settings through Web page:

https://www.tp-link.com/en/support/faq/1402/

May it help and have a nice day!