I've now done this, with a blank server ip in the properties file, the server address is still my iPv4 in the multiplayer menu. it tells me the server is online, I can join and play it, I have a strong connection when on there but my friends can't reach it everytiem.
I forwarded the 25565 port to a 25566 port which is TCP and UDP on VirginMedia, when I launch the server, it binds to the port and the server is up and running for me.
I think this might be the issue. Do I need to allow their IP's through my firewall somehow? I just asked my friend to connect as I shut my firewall off completely but it still made no difference. Hope you can help me out! If it's easier, I can do a teamviewer or something, can't find a lot online that helps with this situation other than the stuff like the ip in the server files being changed or being nothing etc.
Hope this helps!