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You don't normally need to enter anything into the router to enable it's connection to the cable modem, no mac addy, hostname or anything. If you've been tweaking, reset everything back to the manufacturer's default settings and in particular ensure that it can pick up the WAN (internet) connection on auto settings, which get sent to it by the modem.
Remember that you can only have one output connection from the modem at a time. Thus however you connect at present to post this must be unplugged at the modem. The modem must be powered off, and the new ethernet connection formed between the modem and router (WAN / internet port), and a second connection between router (LAN / intranet port) and PC. Only once connected should you turn on modem, let settle, then router & settle, finally PC.