Thank you for the details!
It looks like the issue is related to the theme or something else might be conflicting. If you try to turn off the search plugin completely, and use the search page with any search query, it does not work at all, for example:
– Search for “soja”: https://selsem.rs/?s=soja
– Search for “tvoja”: https://selsem.rs/?s=tvoja
While these should yield at least one result, because there are posts with these titles: https://i.imgur.com/IxNPVTg.png
I am certain that wordpress default search should print out those two items for those queries. Try temporarily switching the theme to the default to see if it changes anything. If not, then turning off plugins one by one should help checking which is at fault.