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Summary of issue

-> There was no USB ethernet adapter appearing in the device manager

-> With DA300 and generic USB-C to Ethernet adapter - nothing appearing

Here are all my actions to fix

-> I went into BIOS > System Configuration and Under Thunderbolt Adapter Configuration - Turn on

* Enable Thunderbolt Boot Support

* Enable Thunderbolt (and PCIe behind TBT) pre-boot modules

(I am not sure either of these fixed issue, but was looking for any non-enabled Thunderbolt / USB BIOS actions)

-> Went to Support.Dell.com
         -> Searched for my device using tag
         -> then in Drivers / Downloads - installed Realtek USB GbE Ethernet Controller Driver
         -> Then Restarted PC

-> NOTE - when I restart with DA300 still plugged in - it didn't work - when I plug-in after restart it works - I now have ethernet connection.

NOW ALL WORKING

I get Both DA300 Ethernet and my generic adapter all working.

I guess the Install noted for DA300 are lacking these steps - hope these help someone

@M__W  Glad it works, although I'm not sure why you had an issue.  Windows 10 has built-in driver support for the Ethernet interface built into the DA300.  Dell's drivers may be newer, but it should have appeared in Device Manager and Network Connections out of the box.  However, I would recommend that you go back and disable those Thunderbolt boot-related options you changed in your BIOS.  The DA300 is a USB-C device, not a Thunderbolt device, so those settings wouldn't be relevant, and having Thunderbolt devices allowed at boot can introduce some security concerns.

jphughan, I was going to - thanks - 

Update - on inspection - the Ethernet is now working - However the 2nd monitor is now not.

I was able to unplug ethernet - restart machine and now monitor is working - but not ethernet - it seems like only 1 or the other is going to work at the same time.

i.e. this is useless with XPS 13 7390 2-in-1

From reading around other posts looks like this the XPS 13 has some serious Port issues - display problems.

Hi, 
I'm having the same problem conecting Dell XPS 13 9365 with Dell DA310u.

It is true that my laptop has built-in driver support for the Ethernet interface built into the DA310, but it only works for the monitor and USB, didn't work with the ethernet. 

I tried "Network reset", set up IP, and it did connect to the ethernet once, but after I turn off my laptop and turn it back again, the signal was gone. I tried to reset my laptop several times but it didn't work.

I also tried Realtek USB GbE Ethernet Controller Driver as M_W, but sadly, in my case, it didn't work.
I hope that there is another way to fix this problem.