for full 4k/hdr & hd audio, nvidia shield is the best, simplest solution.
shield > 4k/atmos receiver > tv = full 4k hdr + hd audio
when you were playing by DLNA, I don’t think you can choose a different audio stream.
there is probably an LG plex client app, which would probably work better than generic DLNA.
the problem with smart tvs they don’t support HD audio (ie truehd/atmos).
so on the CLIENT you would need to choose a non-hd audio format (otherwise plex will transcode and typically cause problems).
this may or may not help you understand what is involved with 4k @ Plex, 4k, transcoding, and you - aka the rules of 4k - a FAQ
