Since the Arris modem is not a Comcast provided device they have no obligation to support it.
Arris does not want you to be able to "fix" the modem as eventually they want do that all via "configuration as a service" involving some fee.
Envision: some one and only one red "check engine" light on the modem. You call, you pay, they fix..... [Full disclosure: cynically minded here.]
Does the following link present the User Guide for your modem?
http://surfboard.com/wp-content/uploads/2014/07/ARRIS_SURFboard_SBG7580-AC_User_Guide.pdf
According to the following link you can access the Arris modem webpages via 192.168.0.1 with the login = "admin" and the password = "password".
Note: Not your model number but hopefully the admin and password are consistent within the product line.
http://m.setuprouter.com/router/arris/sbg6700-ac/login.htm
First see if you can acess the Arris admin pages - see how it is currently setup. E.g., is the IP address 192.168.0.1 (the probable default). Your network devices need to use that address to obtain a IP address that will in turn permit internet access.
Also please run "ipconfig /all" from your computer's command prompt and post the results. Doing so will provide information about your computer's network connectivity. That information is needed to verify that the computer is able to communicate with the Arris and ultimatelly the internet.
You may need to do a factory reset to restore the Arris to default setting especially if they have been changes along the way. But see what can be learned about the current configuration beforehand.