If you have it in an iframe then you should add the parameter allowfullscreen.

An example with a YouTube video in an iframe: https://bramminginfo.dk/hollenderskoven.html#Anker_originalerne

Play the video.

Then click the fullscreen icon and you should see the video pop-out of the iframe and be shown as fullscreen.

The fullscreen can also be used for slideshows in iframes.

On top of the page you can click the fullscreen icon and see the slideshow pop-out and become a fullscreen slideshow,

The fullscreen icon

And for the slideshow

You can also make the object (iframe) expanded

You could also have a video as background.

In row format you specify the video you want to use and you expand it to fullscreen.

Thanks a lot John for all of your suggestions, I will give them a try... Much appreciated ;)