I own a 14” Thinkpad X1 Carbon (5th Generation). I also have a few big monitors that I wanted to connect to the laptop, and the easiest way to do that was via a Thunderbolt 3 dock. I ordered the official Thinkpad dock from Amazon (part number 40AN0135US). When it arrived everything seemed fine, up until I connected the dock to my network via the Ethernet port. A few days after I did that, my network ground to a halt. I was able to verify that the problem wasn’t upstream, and after restarting all the hardware and having the problem persist I finally got desperate and started just undoing any changes that I recently made to the network. It was at that point that I unplugged dock, and after that the network started magically working again. I did some research and found this thread on the Lenovo support forums … apparently I’m not the first person to discover this issue :-( . From the discussion there it sounds like it will only happen when there are two monitors plugged into the dock: one via DisplayPort and another via the Thunderbolt 3 downstream port.
For me this isn’t a huge issue as the WiFi connection to my network tends to be good enough for nearly everything, but it is probably the most bizarre hardware issue I’ve ever encountered.