You cannot search the text in COURSES and have it also search MODULES. In order to find the title of MODULE when searching COURSE the content for MODULE that you want to be able to search must be part of the COURSE post in either the title, content of a custom field if you are searching custom fields.
What this means is that when you update a course post you must create an acf/save_post hook that gets all of the related modules and saves the titles of these modules into some other custom field related to course then you search this custom field.
