C6300 cannot access 192.168.1.1 getting a timeout error.

> took too long to respond
> 192.168.1.1

   I'd expect that.  Not the default address for a modem+router.

> ERR_INVALID_HTTP_RESPONSE
> 192.168.0.1

   That's the default address, so you are (still) communicating with the
C6300, but (obviously) not getting the expected result.

> [...] Reset my modem to the factory setting and I can get in [...]

   Presumably at "192.168.0.1" or "192.168.100.1", or one of the
"routerlogin" names?

> [...] and change the password and update the security questions and
> then when I hit next it goes back to a timeout error.

   "it goes back to a timeout error" or "it goes back to an
ERR_INVALID_HTTP_RESPONSE" error?  If you retry the wrong address, then
a timeout would not be amazing.

   If it works as expected after a settings reset, but then, after some
normal activity, starts misbehaving (when browsing to the same address),
then I'd say that the firmware is corrupt/misbehaving, causing the
management web server to get confused.

   Could be a hardware defect, but I'd try to get the ISP to push
firmware to it (again?), and see if that changes anything.

   Can you see which firmware version it's running now?