Correct permalinks and 404-error

I have a paid plugin that creates a custom type categories and posts in this categories. I want to get URLs like site_name/category_name/post_name. This plugin forms links like this:

• List item

for categories – site_name/category_name

• List item
  for posts – site_name/category_name

I have installed plugin Custom Post Type Permalinks, configured it. When I click on post link, I have a 404 page of theme (not server). But URL is right. In the post edit page the permalink is right too.
Pages shows correctly.
I can’t change any file of paid plugin or theme (it’s already child theme). Can I resolve my problem with .htaccess or m.b. some another way?
P.S. By the way, I’ve already tried to configure permalinks in Settings->Permalinks->Custom structure but result is the same.