@Roqstailor: Bitte setze den Smart nach dieser Anleitung auf Werkseinstelungen zurück. Bei den fünf Sekunden, nehme lieber 10:

Gruß Ulrich

Hi,
Sorry for the English, I don't speak German, but I have exactly the same problem for more than a week now.

It still doesn't work for me, so I can just tell what I did so you don't run the same circles again.
The reset helped temporarily, but not for long. So at the end I finally reached the service via phone, and they sent a technician to check the router the next week.
The technician told me that the router is broken. (Oviously.) And that I have to take it to the store, they will replace it there.
I took it to the store, but they didn't believe me or the technician. They said that it must be fine, it's just the cable that's broken and causing the router to restart every minute. So they gave me a new cable and sent me home, saying if it still won't work I can just bring it back and they will replace it finally.

Obviously the new cable didn't cure the broken router so I took it back the next day, and they told me that they can't do anything with it, and they don't know why the technician or their colleuges told me the other day that they can replace it, I should call the support.
I will call the support on Monday, because I don't speak German, and the English support is only available on weekdays.

So the moral of the story is, don't let them fool you with a new cable, try to get straight to the point and get it replaced. The technician also said that it is a very common problem with these routers, I don't know why can't they change them then...
Anyways, I hope you will have better luck!