The first thing you need to do while you're messing with your router anyway is to upgrade your firmware to the latest version through the Cisco/Linksys website.
Second, make sure your date/time zone is correct (bottom of the first page).
I have found two fixes for this problem that have worked for various people who've had the exact same problem. One is acceptable, and should work for you. The other is a semi-fix.
Every device you use to connect to the internet has a MAC address. Arris modems will only connect to your internet service provider using the MAC address that was used to set up your service (irritating).
When the cable company came out to set up your internet, they connected the modem directly to one of your computers. You need to plug the router directly into the exact same port on the same computer that was used to setup your new modem, and do the following:
1. Go into your router settings (192.168.1.1; username:none ; pw: admin)
2. In the Setup tab, click the sub-heading called "MAC Address Clone"
3. Click the 'Enable' option on this page.
4. Now click the 'Clone Your PC's MAC' button.
5. Now click Save Settings.
6. Wait for the settings to save, and click 'Continue'.
In most cases, this should work, and you ought to have internet access now.
If this did not work, the following is a quick fix:
1. Go to the 'Applications & Gaming' tab
2. Click the sub-heading 'DMZ'
3. Click 'Enable'.
4. Click 'Save settings'.
5. Wait for the settings to save, and click 'Continue'.
If this doesn't work, I would recommend calling your cable ISP and telling them that you need a modem that will work with more than one computer, or you need them to send a tech out who is able to set up the internet through a router. They may give you a hard time and tell you it's the router's fault, and not their problem. Don't back down until they give you a different kind of modem -- you are the customer.