Videos can autoplay on desktop but why can you not get them to autoplay on mobile websites when the page loads? The answer is that the different OS developers intentionally disabled autoplay on mobile devices in order to protect user’s bandwidth. Many data providers charge based on the amount of data consumed, so the OS developers decided it was in the best interest of the user to not have a video automatically begin playing when the page loaded so it would not start racking up data charges. Instead mobile web videos require the user to click them to start.
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I am not sure if you can trigger event inside iFrame...
Why doesn't video Autoplay on Mobile device https://www.aerserv.com/why-does-video-autoplay-on-mobile-devices-not-work/
For your case try this
A solution I've just been working on for autoplaying would be to instantiate the player with an empty video on page load:
var player = new YT.Player('yt-media-player', {
height: height,
width: width,
videoId: ''
});
Then on the click event of opening the lightbox you can load the particular video and call it to play:
player.loadVideoById(youtubeId);
player.playVideo();
Details here