Settings - ‘Show Advanced’ - [server name] - Transcoder - “Enable HEVC video Encoding”
It will probably not show up, if your server hardware doesn’t support HEVC encoding in hardware (which can be caused either by a restriction of the hardware itself, or a missing driver software).
You need to enable detail information in the Dashboard as well.
Proper mp4 files are quite compatible with a large range of client types. If you say MKV are working but mp4 are not, it reminds me of an issue with improperly produced mp4 files, which were missing “interleaving” (that is storing a few seconds worth of video, then the audio, then again video, then audio and so forth.)
If your files are without interleaving, streaming them unchanged over a streaming network protocol can produce issues like those you’re describing above. See Capcut videos playback issues with plex - #2 by OttoKerner
