The DDC has a narrower PWM range.
Most of the speed change happens between like 10 and 50% PWM, so if you changed it above that, it would remain at full speed.
When you set the power slider say to 20 or 30%, do you see the speed change?
You were exactly correct - I am managing to control it now by setting power between 10% and 48%. I'm not sure why pulling it all the way down to 0 wasn't working before, maybe just needed a couple restarts of the system/software/service. Thank you!!