Logging in to arris router

Hello I have an arris dg2460 router and I cannot log into the router settings. I have factory reset it and tried the default admin/password combo but it says invalid username or password please enter the network key on the router. I have entered this too and get the same message. I saw a post around here to use the last 8 of the serial number, this too has the same result. Anyone have this issue or have advice?