You have to disable or delete WP Last Update for it to work.
So I couldn’t figure out the problem using WP Last Modified, however I did get it to work by adding a couple lines of code to the function.php file. Note: this will only work on Divi sites. Hope this helps…
https://www.elegantthemes.com/blog/divi-resources/how-to-add-the-last-updated-date-to-divis-blog-post-meta-data
