Subtitles depend on whether a subtitle track exists in the stream.
Open your IPTV player's subtitle/CC menu and check the available tracks.
If no subtitle track is listed, subtitles may not be available for that content.
To turn subtitles off, select Off in the player subtitle menu. If subtitles are out of sync, adjust the subtitle delay or offset if your player offers it. Testing another compatible player can help identify an app issue.