Be sure you are running the latest BIOS.

IIRC, "Billboard Device" is correct for an Adapter. An actual Hub might be detected differently.

As we found here, not all these cheap chinese adapters work 100% properly.

https://www.dell.com/community/XPS/XPS-13-9300-BIOS-1-0-10-USB-Type-C-to-DP-adapters-no-longer-work/td-p/7575174/page/20

@luis_rr  USB-C to HDMI adapters/cables have to incorporate an active signal converter chip to switch the DisplayPort signal coming out of the system over to HDMI.  And it seems that those converter chips can be problematic and create random interoperability issues.  I've personally seen at least 5 reports of this exact issue here, where an adapter will work fine on another device but not the user's laptop, and in every single case, the "fix" has been to use a different brand of USB-C to HDMI adapter/cable, which ends up working on all devices the user has.  Obviously that shouldn't be necessary, but just as obviously, not everything in the tech world that SHOULD work always DOES work.  The XPS 13 9370 definitely supports video output over USB-C, so if the adapter isn't plug and play, and you've verified that it works with another device, AND you've already updated drivers and firmware, then you probably just need a different brand of adapter.

@Tesla1856  a "billboard device" is a virtual device that can be embedded into USB-C devices.  It exists to generate error/advisory messages that can be displayed on the system in order to give users some guidance if they connect a device that requires optional USB-C capabilities to a device that doesn't provide them.  For example, a USB-C to HDMI adapter only works when connected to a USB-C port that supports video output, which is an optional capability of USB-C ports.  So those adapters might incorporate a billboard device so that if a user connects that adapter to a USB-C port that does NOT support video output, then the billboard device will be used to send a message to the system that can then be displayed to the user, such as, "Hey, your USB-C port doesn't support a necessary capability in order to use this adapter", rather than the user left with nothing happening and no idea why that's the case.  But the billboard device shouldn't be appearing here because the XPS 13 models definitely do support video output over USB-C.

@Tesla1856 Bios is 1.13.1 which should be latest (flashed yesterday).

@jphughan Thanks for the detailed response.

It seems like the conclusion is "it's just an incompatible adapter" and the only option is to return it and to get a different brand. This is unfortunate because there is a wide range of adapters out there with a range of prices (Dell's one is twice the price of this one). I thought HDMI and USB-C are standards (and indeed the adapter works on a different, older, XPS).

So, before I roll the dice again, does any of you know in a new adapter to be sure the next one works? Is there any technical detail to pay attention to?

I appreciate your responses.

@luis_rr  Unfortunately in cases like this where the incompatibility shouldn’t exist in the first place, there aren’t any specs to watch out for, since what you have should work. But I’ve always been a fan of Anker and Cable Matters for cables and adapters. On the Anker side, if you want a multi-purpose adapter, you might look into the PowerExpand 10 Gbps 8-in-1. It’s of course more expensive than a regular adapter, but it does more.

Just for the record. I ended up getting an Anker PowerExpand+ 5-in-1. HDMI and other ports work fine.