Cannot choose external player in android phone

Because i found the subtitles cannot synchronize right in android client/web(it’s normal in pc web), and the client cannot display Chinese subtitles right(all display as little diamonds). so i try to use external player, and in android client(1.0.1) there is an option for external, and i click this option. however, it seems didn’t work, the client also cannot open external player(mx player, vlc).
so i ask for support, how can i open external player? or make the subtitles synchronize right?
thank you.

External player is not currently working. You can follow the GitHub issue for this here.