Can't adjust brightness or update firmware on LG Ultrafine 4k Display (still having problem)

Note: I'm re-upping this question from January 2018 (Can't adjust brightness on LG Ultrafine 4k Display) because 22 other people had the issue but there were zero answers and the thread is now closed.

I can't adjust brightness on a new LG Ultrafine 4k Display I bought in January at the Apple Store. It has never worked.

I am using a 2017 MacBook Pro w/ Touch Bar, running 10.13.5. The screen always just shows the same high level of brightness regardless of light conditions. It will not respond to any of the known ways of adjusting brightness:

1. When using the LG display as a secondary display, I have followed the guidance to press the "<" button on the touch bar and then control-click the small brightness increase/decrease buttons, but that has no effect for me. I can control the brightness on the MPB in the usual way, but the external display brightness does not work.

2. I also tried changing the LG display to be the primary display, but the touch bar on the MacBook Pro still only controls its own screen.

3. Finally, I downloaded the LG Screen Manager app from LG and attempted to use it to download a firmware update. That app could not even tell what firmware version was currently installed on the monitor and refused to pull down any updates. I then selected the factory reset option on that app, but it had no effect. I couldn't find any other controls within the app - YMMV. I have since deleted the LG app.

4. Updates to High Sierra over the past 6 months haven't helped, either.

Has anyone figured out how to actually make this work? Thanks!