I bought Elmedia PRO yesterday but I'm disappointed that subtitles are not shown in the Apple TV, streaming a MKV file. I did some tests:
Streaming a MKV with subs include to an Apple TV (4K) doesn't show subs.
Streaming a MKV with external subs file to an Apple TV (4K) doesn't show subs
Did some tuning (Changing fonts and code) and nothing.
But streaming a MKV with either included or external subtitles to a Samsung TV (DLNA) works perfectly

Is there a different way o tuning to stream subs to Apple TV ?

In order to resolve this issue, please do the following:
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1. Launch the video.
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2. Right-click on the video in the Elmedia Player > Choose the subtitles.
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If the problem still persists, please proceed with the following steps:
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1. While watching a video, swipe down on your remote's Touch surface.
 
2. Swipe right to see the Subtitles menu.
 
3. Swipe down and choose the subtitles that you want to use.
 
 
If it does not help, please contact us via email support@eltima.com for further discussion.

Yep. That solved the issue.
Thank you

Thank you for letting us know. I am glad that the software functions fine now. Do not hesitate to contact us if you need any further assistance.