The <!--more--> tag is a replacement for an excerpt. You want to use the excerpt or <!--more--> tag on you blog/archive page, NOT on single posts.
The <!--more--> tag will not work on a single post like this:
https://igntimes.com/20-amazing-youtube-tricks-and-tips/
However, it should work on the blog (in your case your homepage, https://igntimes.com/. But you only show post-title and image here for every post in the grid, so it will not be visible)
or
in your category archive pages (in your case, for example https://igntimes.com/category/social/. But here the Youtube-Tricks post is only displayed at the top in your special grid, and you also only show title and image in the tiles, so it will also be not visible.)
So please move the Youtube Post in the normal post list under your grid/tiles: (maybe just quickly change the date on that post)
Than you need to make sure that your archive templates are using the_content() and not the_excerpt().
Than you should only see the content above the <!--more--> tag on the youtube post.