192.168.100.1 can you get to this website, its hosted inside the modem, so if not you are not connected to the modem, if you can, then the issue maybe on the cable companies side and so your connection is not getting a valid IP and DNS settings, and thus you cannot do anything.

Also note the modem is not designed to hook something up by Ethernet and USB its a one or the other type of deal, so if this is whats you are trying get a router that has a USB port and tap into that instead.