In Xabber try putting only username into User and 1.2.3.4 into Server field. It only works for me when connecting inside the network (on Wifi) to my local server. Though i think it was working also by specifying real server's name in the Host field, but now it doesn't work so i have disabled Host field. You can also try some other client, like Yaxim (here you can specify username@1.2.3.4 as JabberID). There is also probably a way to test whether a port is open. I know people are using telnet to test if connection can be established, but i haven't tried myself.
5222 should be enough for a remote client to connect. Also check if some firewall isn't blocking connections.