Buy an Nvidia Shield Pro or an Xbox One.
There are two forms of Atmos for home theaters: a) Streaming Atmos, used by Netflix, etc, is Dolby Digital Plus 5.1 + Atmos; b) Blu-ray Atmos, found on discs, is TrueHD 7.1 + Atmos.
TV manufacturers do not let TV based apps pass TrueHD audio over HDMI-eARC to attached sound bars, receivers, etc. Therefore, when you use the Plex app on the TV, the Plex Media Server has to transcode the audio to a supported format.
Some TV manufacturers support TrueHD passthrough for devices attached to HDMI inputs. That is why TrueHD + Atmos works when you use your Bluray player.
Streaming from Netflix works as it uses Dolby Digital Plus + Atmos, which TV manufacturers support passing from TV apps over HDMI-ARC/eARC.
There are two off the shelf streaming devices that support TrueHD + Atmos, The Nvidia Shield and the Xbox One. If you go with the Shield, purchase the Pro, not “tube” model. The tube model has problems with high-bit rate media such as 4K HDR Blu-ray rips with TrueHD audio.
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