One of the most popular related posts plugins for WordPress is YARPP with more than 200.000 active installations. It says it supports Custom Post Types (CPT), but not out of the box. You’ll need to do some extra things in order for your CPT to be supported.
If you have access to the functions that register the CPT, then the solution is simple. You’ll just need to include the argument ‘yarpp_support’ and set it to true when you register it.
If you don’t have access to the code where the CPT if registered, maybe because it’s a third-party plugin that is creating it, you can still try to manipulate the arguments used to create that CPT and add the ‘yarpp_support’ argument. You can use the code below:
You should replace ‘my_custom_post_type’ with the identifier you need and add the code to the functions.php of a child theme or your own plugin. This seems to work well on my projects.
Hope this helps someone!