When I am streaming with an iOS device using 4G/3G, the video suddenly goes back 3-4 minutes and replays what I have already watched but the subtitles don’t go back, they continue, desyncing from the video.
I don’t know if this gives you a clue but if I try to play them with VLC, connecting remotely to the server the video plays fine.
Server Version#: Synology NAS 1.20.0.3133
Player Version#: iOS 7.3.1