Have you read the Multiplayer tutorial series? It covers many of the P2P concepts involved.
According to our measurements, and from other industry estimates we've seen, P2P connections work about 90% of the time across the open internet. Of course if your device is in the 10%, it may seem like "it doesn't work at all", but it does work the vast majority of the time. You can cover almost all the last 10% by hosting a TURN server, which is what most commercial P2P games/apps will do to ensure connectivity. However it's a bandwidth relay server and means you'll have to pay the hosting costs, which is why it's not enabled by default and you have to set up your own.