For more information about Ad-Hoc play, see How to play Ad-Hoc Multiplayer.
Here's a list of known working servers:
Some older Ad-Hoc servers that no longer work, listed here in order to appear in search results:
Some games use network ports that are not allowed for general use on general Linux systems (like Android), where ports 1-1024 are considered privileged.
To get around this, PPSSPP supports adding an offset to all network port numbers. Note, this of course means that for two players to be able to play each other, they must all agree on and use the same port offset. Setting this up can be tricky to coordinate if you don't have a shared chat, or something.