@Bornrich 

Sorry to hear that you have a difficulty time setting up the smart plug, here is an instruction that help to troubleshooting the setup failed problems: 

What should I do if I fail to configure the Kasa device?

If kasa device failed to connect to Wi-Fi network at last step, it's a good step to verify password and confirm correct Wi-Fi password was given in during setup

• How to Verify Router Wi-Fi password? 
  •  Forget router Wi-Fi on your mobile phone and manually input Wi-Fi password to verify router’s Wi-Fi password is correct.   ( Phone Settings > Wi-Fi >tap router's Wi-Fi SSID to forget network or Long press wifi name to delete the Wi-Fi profile).

Or check Wi-Fi password on router Wireless settings page

If you still have trouble setting up the smart device, please provide a photo of error, and let us know at which step the setup fails here, or feel free to reach our support here via Hotline or Chat so they could help you go through the setup process and take a closer look at what's going on.