Hisense is no longer listed as a supported Smart TV by Plex: Which Smart TV models are supported?
The Plex app for Hisense TVs was last updated in Jan 2018: Plex for Smart TVs & Tivo
You may have to use a Roku / Nvidia Shield / etc to successfully play HDR video.
Regarding the log file you attached:
It shows Plex believes the TV does not support 10-bit (HDR) video. This could be due to a misconfiguration of the app or the TV, or that the app itself does not support HDR (see above WRT age, lack of support, etc).
Mar 13, 2020 18:25:43.952 [13392] DEBUG - Thor: Ragnarok - video.bitDepth limitation applies: 10 > 8
Regarding the audio, you do not mention how things are connected. Note that neither HDMI-ARC nor Toslink/Optical support 7.1 audio due to bandwidth limitations. Choose Dolby Digital or dts 5.1 audio tracks when possible to avoid audio transcoding.
