I'm trying to write an android app that will send information to my Pi that is on a different WiFi network. I looked into making it a web server which I'm not opposed to but I don't want to have to mess with firewalls or have to plug the pi into a router. I've also thought about making a server in the cloud that acts as a middle-man between the Pi and the Phone but I'm unsure if that would be a viable option. 

I do have the most recent Raspberry Pi 3.

So what is the best option for communicating data from an android app to a raspberry pi? I'm not looking for a step by step guide just the best strategy for doing this. Any help is appreciated, thanks!