Hi, I've the monitor C34J79x, a Thunderbolt 3 Display with daisy chain capabilities.
What drives me nuts is, that it does not work with any MacBook, apparently. I've a MacBook Pro, late '13 - and use a Thunderbolt 3 -> Thunderbolt 2 Adapter to connect via Thunderbolt. But the Display itself stays black and the standby indication-LED just keeps blinking, like it wouldn't know any better.
You, and I once, thought maybe that causes the problem - since the "source" is at the backward-compatibility end of the adapter, however, the display is recognised with the right model-number, resolution and even the USB-Ports at the back of the monitor are working just fine. For the laughs, I borrowed a MacBook late '19 but the Display did not bat an eye - just the very same.
Also, the cable is fine, works perfectly fine with 2 other LG, Thunderbolt capable Displays. Also tried another cable without any changes in behaviour.
I never read once among the lines of "works on Windows machines only", there is only 1 support file, a driver of some sorts, available for windows.
However, since I doubt that there will be any useful response, I wanted to put that here so maybe someone else is saved from this hassle. Specially, since I had several Displays from LGs in the past and those work just fine - with both, Macs, Linux and Windows machines.