Hi, I'm Mr.med. Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP.
Open your Internet browser and type : http://192.168.1.1 The first step once you being the Quick Internet Setup is to give your RT-N66U an administrator's username and password, Anyone who has access to your Wi-Fi and this username and password ***** be able to adjust settings on your router so be sure to choose a unique username and a strong password.
Let me know the results please
are you trying to just register your ownership with asus ?
i would try waiting until the router is installed and connected to the internet.
It should register itself automatically or at least they will be able to find the mac address of the router when you send the serial number.
it may be giving you a hard time just because they can't see the router existing when you are registering it.
But it could be some other silly problem as well such as a bad email address etc.