I try accessing the GUI to setup guest Wi-Fi but got this message. Invalid user name or password, or your access has been locked due to multiple failed login attempts!

Hello prince

                      Have you changed the login credentials before or this is the first time login. If this is a first time login you can use the default login credentials  

                      On the Login page, enter admin in the User Name field.
                      Enter password in the Password field then click the Apply button. 

                      If that is not allowing you in please perform hard reset on the modem then you can use default login to access modem interface using default login credentials. But after resetting the modem will go to default setup any custom settings will be removed so use the default network name and password to connect to your network.

                     http://arris.force.com/consumers/articles/General_FAQs/SBG7400AC2-Factory-Reset

                     http://arris.force.com/consumers/articles/General_FAQs/SBG7400AC2-Web-Manager-Access