Roamio Plus won't synch over HDMI with new LG TV

I brought home a new LG 4K TV last night. But my TiVo Roamio Plus won't connect to it over HDMI. The TV constantly says "No Signal". I've tried different HDMI ports on the TV, different cables, and pressing the "Resolution" button on the back of the TiVo.

My MacBook Pro displays just fine over HDMI to the TV.

The TV is seeing *something* on the cable because when I unplug & replug it to the TiVo, the TV displays a little "HDMI 1" icon on the screen for a few seconds. But then it goes...

Roamio Plus won't synch over HDMI with new LG TV

Roamio Plus won't synch over HDMI with new LG TV