We tested a lightbox with a picture and an overflow of text on a default install, and it scrolls fine on mobile. So there must be something happening specifically with your site.
Can you confirm some details about what customizations you’ve done? I see on desktop you have the layout-right, but on mobile it’s the lightbox. That is not a built in option for this plugin. Did you set that up intentionally, and if so how? If not then we’ll need to figure out how that is happening.
The other thing I’ll mention is that any js errors coming from your theme or other plugins can cause DA to malfunction. I see a couple errors in the console of that page, so try to get those cleared up to ensure it’s not related. Those errors are saying $ is not defined or not a function. To fix those, where you have $ try replacing with jQuery. For instance change $(function(){ to jQuery(function($){