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If you are using surfboard central application to access modem settings and if this is a new setup please do follow the steps mentioned on the below article if you are using a gateway (modem and router combination) and if the app is already in use and now you are unable to login then you need first uninstall the app, perform hard reset on the modem then install the app after connecting your device to the modem WIFI network then setup the again because for security reason there is no option on the app to reset password or Forgot password. 

After resetting the modem will go to the default setup so all the customization on the home network will be removed you need to set it up  again and you can connect to home network using the default WIFI credentials mentioned on the label underneath your modem.

Please, disconnect the coaxial cable from the back of the device? At the back of the modem, in the middle section you will see a small hole, inside that hole there is a button, and that is the reset button, please insert a paper clip or a tip of a pen and hold it for 15-30 seconds until all your lights go off, afterwards connect the coax cable back and wait until your device fully boots up.

http://arris.force.com/consumers/articles/General_FAQs/SURFboard-Central-Wi-Fi-Cable-Modem-Setup