RE: XPS 9550 not working with TB15 Dock
There are quite a few people experiencing this including myself. I've re-installed the OS 3 times now trying various drivers and just to start again with a clean slate. I have the Dell USB C graphics adapter with VGA/USB/HDMI/Ethernet and that has been working great so I know the port is working correctly on my laptop.
The TB15 dock has been really troublesome but I've not had it work 100% yet (only display works consistently). The ethernet and USB ports will cause it to continuously disconnect and reconnect if you plug anything into them. It gets to the point where you need to power cycle the dock to get the display working again but it'll eventually die again if you have anything UCB/ethernet plugged into the dock.