I checked your site on mobile, I did not find crash issue.
But the image is too big (some ones is bigger than 5000px x 3000px), I think maybe it caused the issue on your device.
Please try to:
1) – download the file( https://www.dropbox.com/s/ziu0extqbpcpr2s/mellow-lightbox-image-size-gallery.php.zip?dl=0 ) , unzip it
– login FTP, go to: themes/mollow/functions/pagebuilder/modules/
– upload / replace: gallery.php
2) Please login wp-admin, go to Mellow / Theme Options, click “Generate New Thumbnails”