If your modem is connected to the internet, but your computer is not getting online, you should check the network preferences on your computer:
- Click on the System Preferences icon in the System Dock, or click on the Apple Menu and select System Preferences.
- In the System Preferences window, click on the Network icon.
- In the Network Preferences window, click on the Locations menu and select Automatic.
- Click on the Show menu and select Built-In Ethernet.
- Click on the Configure menu and select Using DHCP.
- Make sure all other fields are black and click the Apply Now button.
- Close the Network Preferences window.