Can't adjust brightness on LG Ultrafine 4k Display

I can't adjust brightness on a new LG Ultrafine 4k Display I bought last week at the Apple Store. I am using a 2017 MacBook Pro w/ Touch Bar, running 10.13.2. 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.

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