Just as an FYI, I had an Arris TM402P that had that problem. When I would go to the diagnostic page (192.168.100.1), it would just time out. It was that way for months.
However, on the 8th of October, it just started working.
I just recently upgraded that modem to a D3 (Arris TM722G/CT), and the diagnostic page still comes up without a problem.
As far as I know, it was a firmware issue. I don't know what the plan was to roll that out to the effective hardware. You may want to ping NetDog
, as he was the one I was working with at the time. I also told him the day it started working, and he seemed really excited about it.
--Brian