Hi @ABO896. Welcome to our Forum here. If the LEDs on all three of the Experia WiFi extenders are blue, this means the devices are connected to your Experia Box correctly and have all obtained an IP address. The first time you use the KPN Thuis app to install your extender, the app asks you to create a password. It's the same password you use to log in to your Experia WiFi. Have you tried that yet? If that doesn't work, I’d advise you to reset your Experia Box and reset your Experia WiFi extenders. One of the extenders will become the so called “master device". You should use the password of that device to log into the app. 

Hi @ABO896. I haven't received a reply from you anymore. I hope this means the issue's been resolved. If there's anything else I can do for you, please let me know.