It is not intuitive

Hello,

thanks for the review.

On the plugin settings page there are many links to the documentation pages including to the Quick start page:
https://adinserter.pro/documentation/quick-start

Each insertion position is clearly labeled, you only need to choose one:
https://adinserter.pro/documentation/automatic-insertion

Of course you can choose where the code will be inserted, this is the main feature of the plugin:
https://adinserter.pro/documentation/code-editing

However, if some ads do not appear then this has nothing to do with the plugin as it only inserts ad codes. Please check how Ad Inserter works:
https://adinserter.pro/faq/how-ad-inserter-works

To diagnose the insertion issues the plugin provides many debugging functions:
https://adinserter.pro/documentation/debugging

And if you need help with the settings you can also open a thread in the support forum:
https://wordpress.org/support/plugin/ad-inserter/