I called the Dell support line to try and establish how to network boot. Unfortunately, after being passed around several times the guy I ended up speaking to had not got a clue what network booting was and I ended up just putting the phone. Only had to call Dell Support a few times and really not very impressed.
Basically the D6000 dock does work with the 5290 2-in1 tablet, however, you need to be in Windows and install/update the drivers for it and voila, a working dock. Unfortunately, this still doesn't help with networking booting so I have had to resort in wasting a lot more time in manually building it and should it ever need rebuilding, it will need to be factory reset and set up manually again.
I can only assume, as like the Microsoft Surface, there may be a USB to RJ45 dongle that will allow network booting from PXE/UEFI.
Very disappointed Dell support could not give me this information.
I was able to resolve the problem by using a Thunderbolt WD-19 Dock. Dell also sells USB-C adapters to that state they allow you to PXE boot on these tablets; which I've used in the past successfully, but have since misplaced.
It took me a while to figure it out, I tried using a Belkin brand USB-C adapter, regular USB NIC and D6000 dock, nothing worked at first until I used the Thunderbolt WD-19 dock.