Thank you Shibby!
Now I found this guide:
(sorry, wasn't allowed to post links)
There I read:
No need for the Asus recovery software as you can just do the following.
Place the router into recovery mode.
Open web browser to 192.168.1.1 and the miniweb server will come up with options to upload a firmware, reboot router or clear nvram.
You have to set their computer to static ip 192.168.1.2 and subnet mask 255.255.255.0 in order to access the miniweb server on RT-N66U (192.168.1.1).
Somebody tried this way with success?
But for being on the safe site I will do it this way:
1. Plug into the router with a cable and set your PC to have an IP address in the 192.168.1.1/24 range, but NOT 192.168.1.1. So anything from 192.168.1.2 to 192.168.1.254.
2. Put the router into “rescue mode” by doing the following:
A. Press and hold on to the reset button with the power cable disconnected.
B. Plug in the power cable and turn on the power button so that the RT-N66U turns on.
C. Keep the reset button pressed until the power led begins blink slowly. The router is now in Rescue Mode.
3. Start the firmware restore utility, browse to the downloaded firmware file, and start the upload. When you are warned that this is not official firmware, click OK. The firmware will upload and the router should restart automatically.
4. Change your PC’s network settings back to DHCP and connect to the Tomato interface and clear the NVRam as suggested above.
I think the router arrives monday! <3