@Triin 

Please check if there is any Wi-Fi button on the extender powerline unit, you could turn on Wi-Fi by pressing the Wi-Fi button for 5 secs as your device user guide.

If there isn't a Wi-Fi button, double-check the model of your power line kit and see if its a Wifi powerline kit, or Wired powerline kit.

Please check if the model start with TL-PA, or TL-WPA-? 

The 'W' in the model means its Wi-Fi powerline unit and has the Wi-Fi feature

In Case1 of FAQ How to choose a TP-Link Powerline product, you can find out how the Wi-Fi powerline kit looks like.