Pinging yourself is not a very represative test of your network setup. Better to install a ping app on your phone and try to ping your desktop. After that, you may also need to configure your router to let Mnemosyne's port through, which is a different
one from the one ping uses. Also, always start by having both machines on your home desktop, making sure the sync server is running on your desktop, and double checking port, ip, username and password on the phone.
Good luck!
Peter