AirPort Extreme to extend my current Wifi and let me connect a PC over Ethernet not working

My intention is to use it in the same way as the left AirPort Extreme as shown in the picture.

So connect to the current WiFi and then connect a PC via Ethernet. Would I really need 2 of these devices? Or should it also work with my current WiFi modem?

At the moment my Apple AirPort Extreme is not extending my Wifi at all.

It gives the error: This AirPort base station is set up to join a specific wireless network that can't be found.

While my pc that stands right next to the AirPort Extreme connects perfectly over the Wifi.

I wonder, is it really necessary to have 2 of the AirPort devices?

Or might my device be faulty?

One thing though that wonders me as well, while setting the name of the Wifi network it is not showing me the available networks. I needed to type the Wifi network name, but still it's not working.

Does anyone have experience with setting up the AirPort extreme as a Wifi extender and than be able to select from the available Wifi networks the one that you want to extend?