Since the online light is OFF on the modem we would suggest you to perform hard reset on the modem and check for the LED lights if still online light is off then login to the modem interface to check the WAN IP address and also try re activating your modem with your service provider.

To perform hard reset please, disconnect the coaxial cable from the back of the device? At the back of the modem, in the middle section you will see a small hole, inside that hole there is a button, and that is the reset button, please insert a paper clip or a tip of a pen and hold it for 15-30 seconds until all your lights go off, afterwards connect the coax cable back and wait until your device fully boots up.

After resetting wait for the modem to reboot completely and on the device connected to home network access the modem GUI

1. Launch a web browser, such as Internet Explorer, Firefox, or Safari.  Enter http://10.0.0.1 into the address box, and press the Enter key. 
2. The Login page will appear.
    
3. On the Login page, enter admin in the Username field.
    
4. Enter password in the Password field then click the Login button.

and select Gateway > Connection > Status check for the WAN IP in Xfiniy network.