You will need to call o2 and report a fault.

Which router do you have and which lights are showing / flashing on the router?

I use O2 homebox 9741 and Only Internet signal is blinking…

Is there a hotline where I can call in English?

@Arahan89 

welcome to our community. 

At first you’d better install our o2 My Service App to analyse the default connection. Language can be switched to english.  If there is something wrong you get a pre-ticket ID and the direct line to our technical department. But it is unsure to find someone speaking english from the start. 

The second option is to call the English customer service  Mon to Fri  from 10 am to 6 pm. Call 089 6666 300 81. But please be aware that not every employee is familiar with DSL Tec.

Greetings and thanks to @bs0 for assisting.