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You don't normally need the username and password for a cable based connection, other than when you initially register the cable modem. Once the modem has ben registered, it's normally sufficient to power it off, connect your new device, i.e. router in place ofa computer, power on modem, power on router, then computer(s). If you've been fiddling with the router settings, try a restore to factory settings before going any further.
You did buy a cable, not ADSL based router didn't you?