1. How is this gallery created? Is it a plugin or your theme functionality?
2. Does this gallery reside in post countent or somewhere outside?
3. Did you try using the Force lightbox functionality maybe? (in RLG settings)
Basically the gallery is missing data-rel=”lightbox-gallery-n” parameter (where n is a unique gallery number)
Most probably it can be added via filter hook or gallery template adjustment.
If the Force lightbox solution won’t work for you please share ftp and admin crednetials to your site (using private reply) and we’ll try to handle that for you.