Hello @Peppe81 

I recommend reaching device manufacture directly so they could provide  more professional troubleshooting suggestion with your smart switch.

You may check below settings on tp-link extender(AP mode): 

1. Ensure you are using correct WiFi password when set up your smart switch, it is better if you could test that wifi password on a phone, e.g Forger wifi on the phone and reconnect to it to check password is fine 

2. Make sure other devices could connect to tp-link extender / AP