I can finally reports that I have cracked it. The only way to be sure that your DX4000 is not susceptible to ethernet failure is to setup this USB nic while you can connect, load the drivers and make sure it works. If the sentinel onboard nic fails you are still able to access the server through the usb.
StarTech.com USB 3.0 to Gigabit Ethernet NIC Network Adapter - 10/100/1000 Network Adapter - USB to Ethernet LAN Adapter - USB to RJ45
https://www.amazon.co.uk/gp/product/B0095EFXMC/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
Thanks for all your help Gramps. I would advise every DX4000 user to do this.
