By default, the platform supports iframes from Youtube, Vimeo, Facebook, and SoundCloud. The rest of the platforms that provide sharable content through iframes are blocked, for security purposes, by the platform.
The way to bypass this is by adding a comma-separated list of URLs that provide the iframe content, to the advance setting:
Once the URL of the website providing the iframe has been added, the content should immediately show up once the iframe is shared on the site.
Important note, sharing content through iframes should be supported by any froala editor of the site.