Hello Chantela

                    Admin and Custom Password Login Credentials
The custom password for the admin login is created during the first time login to the SBG8300 Web Manager.  The Gateway Home Network Wizard loads to customize the admin password, Wi-Fi credentials, and the time zone.  It is required to complete the Home Network Wizard in order to access the Web Manager.  When accessing the Web Manager the Login page will ask for Username and Password. 

                   ARRIS Account Email and Password Login Credentials
The email login and password is created when the SURFboard Manager App is used for first time setup.  It is required to register an ARRIS account with an email address and password when setting up the ARRIS SURFboard Manager app with the SBG8300.  When accessing the Web Manager after the ARRIS SURFboard Manager app is setup, the Login page will ask for Email Address and Password.   

                   The SBG8300 Gateway has two possible login credentials, admin and custom password or ARRIS account email and password.  Only one client device can login to the Web Manager at a time, otherwise the second client device would be logged in and first client device logged out.  If the password is lost, performing a Factory Reset will erase any configuration changes that have been made and will require the gateway to be reconfigured.

                    You can try changing WIFI channel for 2.4 network to channel 1 so that will allow you to connect all your third party devices to the 2.4 network. Please follow the steps mentioned on the link below to change the WIFI channel.

                      http://arris.force.com/consumers/articles/General_FAQs/SBG8300-Changing-the-Wi-Fi-Channel

Let us know if this helps.