Latitude 5510 and WD19 dock network cable / 5510 solo network cable (RJ45)

Since buying the laptop I have problems with the cable network connector (RJ45).
Problem 1: In the dock it fits fine, however I get no connection via the dock. Display and USB worked properly.

Problem 2: The same network cable plugged into the Laptop, does not fit properly, I often loose contact.
All solutions of DELL already tried, support had no idea and the device was already three times in repair.
Latest drivers installed, BIOS settings checked.The device only works with WLAN. One could get crazy.

A second device of the same type also does not work. I am surprised, in view of the massive problems, that others have apparently not yet noticed. Any solutions?