Do the link lights on the modem and next to the ethernet port on the computer light up when the cable is connected and both devices are powered on?  If not, does wiggling the cable or pushing it in result in lighting the link lights.   If not, and your are sure the cable is not defective, then it sounds like a bad RJ45 connector in the ethernet port on the E310.  You will have to call Dell Tech Support for service.

 

Steve