Unable to customise a 'Lightbox'

Adding a 'Lightbox' to an image does not seem to be working as expected. I have attempted to add one to an image (Clicked Image > Image Caption dropdown > Image Link), selected 'Lightbox', named it and clicked Ok.

The 'Lightbox' is added successfully and I can bring up the settings menu (Green Cog Icon) but other than adjusting the Height, Width, Area Colours and Padding, I cannot seem to add in any text as well as altering the CSS.

Attempting to edit the CSS (Edit Region CSS) brings up the editor at the bottom of the screen as expected, but you cannot choose to pull in either of the Available element selectors or Save.

Can you please advise?

I am currently using the 'Upfront Builder' plugin which is completely up to date as well as the 'Fixer' child theme.