Convert mkv to mp4 for Apple TV

Install ffmpeg and MKVToolNix.

Check mkv tracks info.
mkvinfo -g

Result:
video: H264
audio: dts
subtitle: srt1
subtitle: srt2

mp4box: Adding Subtitles to mp4 Movies for Apple TV

Download and install mp4box on windows.

1. srt file use UTF-8 encoding

2. mp4box -ttxt origin.srt

3. try to change the font size

-<TextStreamHeader width="400" height="60" ...

+<TextStreamHeader width="video.width" height="video.height*20%" translation_x=0 translation_y="video.height*80%"
4. mp4box -add origin.ttxt:hdlr=sbtl:lang=eng -add origin.mp4 output.m4v

enjoy it...:)