Using 12K URSA Mini Pro with Mini Sync Generator

Hi folks. I have a Blackmagic Mini Sync Generator that I'd like to connect to a pair of Blackmagic 12K Ursa Mini Pro cameras, to use the tri-level sync function. The camera is recording at 12k 60fps. From what I've read in the 12K Ursa manual, I can use either the SDI-In port, or the Ref/TC port to accomplish this, and a "REF" should appear next to the timecode display on the Ursa when it's working. I have tried both SDI and Ref ports, both "Program" and "External" settings for the Ursa reference input, both 1080i60 and 1080p30 from the sync generator, both 60fps and 30fps recording modes on the camera, and every permutation of the above. Nothing works. I never see "REF' appearing on the display. What am I missing?

My goal is to "genlock" two Ursa 12K cameras running at 12k 60 fps, so that they capture frames at exactly the same instant. If there's another way to do this than what I've mentioned above, I'm all ears.