What is the wifi card model?

Here’s the guide to get to it: iMac Intel 21,5" EMC 3068 Airport/Bluetooth Karte austauschen Presently this part is hard to find! iMac 21.5"/27" Airport/Bluetooth Card Apple P/N 661-07505 you’ll also need the heat transfer putty.

Are you sure that’s the problem? This is a custom part made by BroadCom for Apple.

I would do a bit more diagnostics before opening the system. Are you able to see the WiFi hardware under the System Report? What do you see in the menu when you press the Option key when you click on the WiFi icon on the menu? Take a snapshot of the grey area and past it here for us to see Bilder zu einer vorhandenen Frage hinzufügen.

If the system was already opened then it maybe the antenna plates got damaged as they are just behind the display glass.