Essentially, Small Screen Mode displays thumbnails and main images on separate pages (to allow more room for the main images to be displayed on small screens).
In Small Screen Mode, the gallery title is displayed only on the thumbnail pages (above the thumbnails) and not on the main image pages.

Your gallery sets showSplashPage="NEVER" which is why the Splash Page is skipped when you view your gallery on a mobile device.
Also, your gallery sets autoPlayOnLoad="TRUE" which is why the thumbnail pages are skipped and the gallery immediately displays the first image.
Incidentally, your gallery sets buttonBarPosition="NONE" so the button bar is not displayed and there is no thumbnail button to allow visitors to visit the thumbnail pages.

If you want the same experience on mobile devices as in desktop browsers, then you can have your gallery displayed in Large Screen Mode in all circumstances by setting screenMode="LARGE" (in JuiceboxBuilder-Pro's 'Customize -> General' section).

I hope this helps to explain things.

They work fine in a desktop browser screen even if I resize the screen down to mobile size.

But when I use a mobile (Galaxy S20+) - they are not there?

Sample Page - https://www.bda4sale.com/slideshows/slideshow.php?pid=1

Any ideas??