Custom slugs for categories and adverts

Hey guys, I need to add custom slugs for categories, adverts and pretty much everything I can since the site isn’t going to be in english.

I have come across a snipped called Custom Slugs for WPAdverts (https://github.com/simpliko/wpadverts-snippets/blob/master/custom-slugs/custom-slugs.php) but it doesn’t seem to work or I’m doing something incorrectly, I installed it like any other plugin and changed the slugs accordingly, they do take me to the new urls when I click on something, however it gives me a regular 404 error, always.

This is how I have the plugin: https://pastebin.com/tJjWCbTc

Any help would be appreciated.