Hello @wiebbelling 

Try a factory reset again by pressing the reset button on the back of the Zyxel for about 10 seconds using a small object.

@Waqqas tried again, same results: cannot login or access my router through T-Mobile Thuis customer page. Internet connection works, but cannot access router to change some settings. What should I do?

Hey @wiebbelling, smart that you are looking for different perspectives. I took a deep dive into your modem, and it seems as if everything is adjustable again from your side. The settings page of your My T-Mobile Thuis account will definitely work. Feel free to ask the Community and I any other questions you might have about the modem, and have fun with our internet!