Adding subtitles to post on masonry grid

I'm using GP premium and on the home page I'm using Ultimate Gutenberg Addon for the masonry grid. I want to add a subtitle underneath the title. I've followed David's advice from another thread, where he advocated downloading WP Subtitle and adding a hook element. That didn't work for me.

The content box in the grid masonry editor looks like this:

Is there anyway I can modify the plugin to show "Show Subtitle"? Or is that even necessary?

Thanks