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.