Hello, I have managed to put the option to sort by date in the desktop version, but I need to put it in functions.php so that it also works in the mobile version. What code should I put in order to order for the latest publications and the oldest publications?

https://leales.org/search

This is default sorting of osclass, not sure what you want.

The order of the publications in the searches is by price. I want it to be sorted by the oldest or newest items, in the mobile version.
You can see in my desktop version how we have placed the order of the publications: by oldest or newest.
I've seen that I need to have to modify it in functions.php

No by default it is ordered by publish date descending.