So you have devices connected to modem WIFI network without internet? - If Yes

Please try accessing modem settings page or GUI and check for the WAN IP address to confirm whether your modem is receiving signal from the service provider or not.

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Web-Manager-Access

did you check with the ISP to ensure you have it activated to your service?

After activating the modem with Internet Service Provider(ISP) still unable to connect internet try with a re-provision with ISP .

For more information reach out to arris.com/consumersupport