I have Dell XPS 13 7390 and Dell Thunderbolt Dock- WD19TB  but when I connect an ethernet cable to the dock or to the laptop it will disconnect from the internet for a few seconds and reconnect regularly.  WiFi works ok. 

• I have used different cables and still disconnects. 
• Dell has replaced my docking station twice. 
• Dell has replaced the motherboard.  
• The router has been replaced.  I have replaced the router
• Dell has updated all of the drivers. 
• I saw this on a different thread in the Dell Community which said to do the following - I have disabled Energy Efficient ethernet and green ethernet and wake on Magic Packet

I have tried the ethernet with a different computer and it works fine.