Hi - this is a long shot maybe, but the following is my story. On my first 645 I kept getting page not found when I tried to access the router on 192.168.0.1. I took it back and got a new one ... which promptly did the same. I always had to reboot then I could access but later I couldn't.
Somebody suggested that DDNS might be causing a problem. I went through every device on my network disabling every mention of DDNS on every device. Eventually I disabled DDNS on one of my IP Cameras, it promptly through a wobbler - I couldn't access it. But I could now access the router. After a while the IP camera started working again and now, over a week later harmony has returned to my network. No problems with either camera or router or anything else. I'm not 100% sure, but the learning is that it appears other devices may be able to cause problems and interfere with accessing of the web interface of this router. Good luck.