Can't Reset NBG6604

Hello I've got a Problem with my NBG6604 I can't reset my router and I can't get access to the administration side. I pressed the reset button for more than 20sec and nothing happens. 
The power led is flashing in a rhythm of 3 sec and the 2.4 GHZ WiFi led is on. Is there any possibility to reset the router?
Thanks for all answers
Kind regards

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