First make sure that your server.properties folder contains your ip4 adress, usually looks like 192.168.1.3. Then after that go to whatismyip.com and give your friends that IP adress to log onto. If thats still not working i would try changing your port forwarding settings from 25565-25565 to 25564-25566. That is what i did and my server works internationally just fine.
I did everything correctly and it wont work can someone help?