XPS 15 9550, Thunderbolt NVM firmware issue with Thunderbolt 3 to 2 adapter
Because I didn't liked USB 3.0 ethernet chipsets and USB-based ethernet cannot bear some network-intensive tasks, I wanted to use PCIe-based Thunderbolt ethernet controller on my XPS 15 9550. As there is no Thunderbolt 3 "PCIe" ethernet adapter (except docks), I chose to buy Apple Thunderbolt 3 to 2 adapter and Thunderbolt 2-based ethernet adapter.
I plugged it into the XPS 15 9550 (tested with BIOS 1.1.19, 1.2.14, 1.2.16 and latest Thunderbolt firmware with NVM version 16), and the PCIe device did not pop up. Instead, only USB billboard device is appearing saying that "Alternate Mode configuration not attempted". Both Windows 10 and Ubuntu 16.10 failed to detect the device. I moved this configuration to MSI GS63 laptop, and it detected ethernet controller successfully (that laptop utilizing NVM firmware version 18).
It seems that other vendors are affected with lower NVM firmware version involving setup with Thunderbolt adapter: see the case of Lenovo too. Do you have planned Thunderbolt NVM firmware update for XPS 15 9550? Looks like XPS 13 9360 is using newer Thunderbolt NVM firmware version, though.