If your new modem is an ARRIS SurfBoard S33, the attached documentation should give you the login instructions along with the Username and Password?

Mine specified navigating to IP 192.168.100.1 and using Username:: admin ,with Password: Last 8 digits of your units Serial Number.

Hope this helps.

the default password tends to be either

password (all lower case)

or 

the last 8 of the serial.

but you really don’t need those to use it. You can contact your isp to have them initialize it and then just connect a router to it