Hey Agata,
some words upfront: I am in no way affiliated with vf, just a customer myself.
Nonetheless I went through a similar situation when I signed up for a vf cable contract several years ago.
The fritzbox was unable to sync with the cable network (indicated by the flashing power/cable LED) despite the fact that vf already provisioned the connection.
If your access to the cable network is already activated (date should be stated on the "Auftragsbestätigung" - contract confirmation letter) it may well be that the wall outlet you are using to connect the Fritzbox to is protected by a "cable filter" which is intended to block some of the "sending data channels" to ensure better TV reception especially if the wall outlet has not been replaced by a modern "Multimediadose" (wall outlet) which technicians usuall do if they do the setup for you. In my case, the previous owner had his Fritzbox connected to a different wall outlet in a different room and all other outlets were protected by a net filter for better TV reception at the splitter (the little box where all tv cables in all rooms come together). Once I removed the filter (a little "plug" that sits inbetween the splitter connection and the cable going to the wall outlet) the Fritzbox was able to connect to the cable network just fine.
The same may not be applicable in your case but it may well be worth checking.
Hope you get online soon.
Yours,
Daniel
