First, try a more recent version of Ubuntu. Next open a terminal, type in "ping www.google.com" and see if there is a response. If there isn't then type in "ping 18.104.22.168" and let us know if there is a response. If there isn't a response to the second one, then you aren't connected, if there is a response to the second one, but not the first, then the dns server of your isp isn't doing it's job. You could also try installing knetworkmanager, or whatever gnomes counterpart is, and trying to get it to configure things for you. It's possible that a distro that old just isn't doing it on it's own, as a more modern distro would. Also, I'm sure the support drones you spoke to told you to disconnect the modem for 2 minutes, then reboot your computer, that can work sometimes, but makes sure that there is no power to the modem for over 2 minutes, don't just turn it off, disconnect it from the wall, pull the plug.