Fortunatly, there is a way to connect to other players lan servers changing ip or something, but playing on-multiplayer this way is pretty crappy becouse of laging server and worlds not loading propertly for everyone
There could be a way around this, something like hamachi, but on the pi. Anything to "trick" it into thinking they are on the same LAN and connect it over the internet
The Raspberry Pi isn't capable of hosting a Minecraft server at all. Because it has 256mb-512mb that is barely anything. In order for it to be lag free you need to turn the view distance really low and have everything to false and etc. You can have about 1-2 people any more and it will lag. I tried it out my self.