Help with connecting

[Famlit]

Hi guys, I am relatively new to Cuberite. I have a raspberry pi and some friends wanted to play minecraft together so I said that I would set up a server. I have so far successfully installed it on my pi 3 and I can access it from a computer that is on my home's wi-fi. I gave them the ip that I got by typing in sudo hostname -I or whatever it is and they tried connecting but they couldn't. I guess the big queestin that I am asking is, how do I set it up so I can give them an IP and they can direct connect to it from their computers in their houses? I have tried and tried and have been unable to do it. Thank you