Jerboa search only finds communities that at least 1 person on your instance subscribed to, to find new communities from other instances easily I like to use https://browse.feddit.de/

Then when you find a community, go to the web version of your instance (don’t worry it’s (mostly) mobile friendly) and type !name@instan.ce (don’t forget the !) Then you can subscribe there. Close and reopen Jerboa and your new community will show up in the list. The Jerboa devs are working on fixing this.