Ethernet issues on WD19/WD15 dock

Hi All

We have a strange issue with our Docks Realtek ethernet network. we have a problem where if a computer is on Wi-Fi at meeting and a user comes back to its place and docks in. the ethernet says its connected and connected to the internet, but there is no connection what so ever. we cant ping anything, not even internal IP's. and we can se that it uses the Wi-Fi IP, even though it is turned of. this also happens if a user goes to lunch and comes back, then the connection is dropped again

The models we have problem with is: Latitude 5400/10/20 and 7300/10/20 and 7400/10/20 

we have all the latest Drivers and firmware updated the dock through Dell Command Update.

We have tried a lot of testing with USB power settings etc.

Hopefully some of you know about this issue and can help.