Can't See Mac Mini DTK on My Monitor

I know my Mac Mini DTK boots because I hear it ask if I want to turn on VoiceOver. But my monitor never displays it. The monitor is an LG UltraFine 24" (24MD4KL-B), which the DTK Release Notes specifically say is supported at 4K using DisplayPort alt-mode. But I have connected it both with an Apple USB-C cable and with a Thunderbolt-2 (Mini DisplayPort) cable and USB-C/Thunderbolt 2 adaptor, both of which should support DisplayPort alt-mode, but I get no video -- the monitor remains dark. My old Apple LED Cinema Display 27" doesn't work, either. Any suggestions?