Are you trying to access modem setting through the modem GUI or using surfboard central app?

If you have surfboard central app are you able to login to the application?

If you are using a GUI please try on different browser or on different device also try restarting the modem then try also clear the browsing history cookies and catches then try accessing interface.

Also, adding to the post you had enquired about the warranty period.

In general, when you purchase a modem/router from Arris the warranty period will be for 2 years from the date of purchase and one year if it is a Refurbished unit.