Plex does not support ATSC 3.0 broadcasts. They use AC-4 audio and Plex does not have a license from Dolby to handle the audio stream.
It is unlikely Plex will ever handle such broadcasts, as broadcasters are increasingly encrypting them with DRM. Given the current rules and regulations, Silicon Dust and other stand alone tuners cannot decrypt such channels.
Also, I’ve not yet heard of any station broadcasting in 4K. Most rebroadcast their ATSC 1.0 channel (720p for KDFW).
Two threads on the topic with additional details: ATSC3.0 Broadcast and AC-4 Sound and Atsc3 Support.
