Interesting. Could you post a link to the dongle?

Not sure what the other guy uses, but I use this Anker with my LG OLED to get RGB 4k@120 HDR with my M1 MacBook Pro:

https://www.anker.com/products/a8317?variant=42329259475094&...

On mine, Apple TV+ (the official app as well as Safari) will refuse to play 4K through a similar adapter (VMM7100) on an OLED C2 42 from Cable Matters with the latest 120Hz supporting firmware. I assume it is because HDCP is broken. It works fine with the Mac Mini's built in HDMI. Frustratingly there is no great way to debug this, but if you open up Safari and look in the network tab, you can see the resolution of the video being streamed.

Does your adapter work at 120Hz without updating the firmware? If it does, does it support HDCP?

Ah yeah you just reminded me of the rabbit hole I went down a couple years ago.

Yeah I had to flash the firmware on the Anker dongle, following this guide: https://forums.macrumors.com/threads/dp-usb-c-thunderbolt-to...

I just tried streaming 4K through the Apple TV app on my M1 Macbook Pro and no issues, so I'm assuming HDCP works.

I'd go with a caldigit TS4 + their hdmi/dp active adapter.