1. Unplug the power to your modem.
  2. Follow the Ethernet cable from the modem to the router. 
  3. Unplug it from the router.
  4. Plug this end into your computer.
  5. Plug the power back into your modem and let it start up. 
  6. Restart your computer.
  7. Attempt to connect to the game.

If this succeeds, it means that your router is unable to maintain the connections mentioned above. Your router may need a firmware update or it may need to be replaced.

For information on updating your router's firmware, contact your router's manufacturer.