On an iPhone, subtitle and caption features depend on the specific app or streaming service you’re using. Apple’s Live Captions is a built-in accessibility feature, but it does not apply universally across all video streaming apps or when content is cast to external devices like Roku TV. Custom subtitle support is usually controlled by the streaming app itself, and some apps may not support importing or displaying user-added subtitle files. To use custom subtitles, check whether the app you’re streaming from has a subtitle option, or try playing the video directly on the iPhone with a compatible media player that allows custom subtitle files.