Position in category page

Insertion between paragraphs is normally only possible for post/static page content:
https://adinserter.pro/documentation/automatic-insertion

This is theme dependant.

You can still try to use insertion before/after HTML element:
https://adinserter.pro/documentation/automatic-insertion#before-after-html-element

Since my category pages are full of content do you know if there is an alternative way to insert ads?
Should I contact The Theme Developer?
Or do you know another plugin which can add ads in category pages?

Please check my post above.

You can use debugging function Show positions to see available positions for automatic insertion:
https://adinserter.pro/documentation/debugging

To insert elsewhere you can use insertion before/after HTML element (you need to know some HTML/CSS basics).

Another way would be to customize theme file for category pages by adding custom hooks
https://adinserter.pro/documentation/automatic-insertion#custom-hooks

If post content on blog pages (like category page) is not processed by the_content filter, then the plugin can’t insert there using standard automatic insertion.

I would suggest to ask theme authors for more info.