Tubio app on your mobile device to cast to LG SmartTV when TV's "SmartShare" (tv app) is enabled. it's better than now defunct (June 2017) Youtube App.
https://play.google.com/store/apps/details?id=com.aesoftware.tubio https://itunes.apple.com/gb/app/tubio-cast-web-videos-to-tv-chromecast-roku/id934339109?mt=8