Configure your router and redirect ports
Most of your devices access the Internet through a router, probably given by your ISP. The router has a public IP address, making it reachable from the outside. We must learn how to tell the router to forward requests from the outide to your server (not anything else) depending which port number is used.
You can imagine your router as a big wall when seen from the outside. In this wall, there are doors : we call them "ports". When someone knocks at the door 80, he want to see a web serveur. Your router has a configuration line for this cas and will redirect the visitor to your server internal IP.
Most of the doors will remain closed, because you didn't configure a redirection for them, and that's fine to keep your other equipments safe.
Your router is a GATEWAY.
Understand that if your server is reachable with IPV6, no redirection is required : its IPV6 is routable and reachable. That's why it is important to always configure the firewall on your server too.
To configure your router, it might have a web interface reachable using a browser from a computer connected on your local network. Some ISP provide a configuration tool on their web panel. It depends, you'll have to find out or ask your ISP how their router work.
Most of the time, you can try these URI in a browser :
Probably, a username/password will be required. Ask your ISP, or try "admin/admin" or "admin/password", or look what's written on the router.
Here are more clues :https://www.wikihow.com/Configure-a-Router