SimplyRFiD

This is a very tricky issue to troubleshoot with the proper tools for the job. 

If this is a patch cable, used to connect a device to a switch or router, you can try replacing the cable with a cable you know works (perhaps from a device currently connected to your network, or simply a fresh cable). If the device starts to communicate with the network, you've identified that cable as having an issue of some sort.

If the cabling is internal to your facility (i.e. inside the walls), then it becomes quite a bit more troublesome to determine whether or not cabling is the issue. The best way is to get an Ethernet Loopback Jack to test the line. Should you need more advice on testing cabling, let us know and we'll be happy to give you more suggestions!