I recently tried to make a Minecraft server for 1.13.2, I'd say I'm quite experienced with making servers as I have made many successful ones running off my own computer back in the day!
I have successfully port forwarded my 1.13.2 server and I can even join the server using my iPv4 IP. My friends, however, cannot join as they put in the external IP and they just can't join? It literally says "Can't connect to server". I'm very confused, I've seen a lot about "firewalls" blocking it but I did remove my firewall on my computer and also on my internet, tested it and still they couldn't join the server.
I've tried the same thing on my brothers' computer, exact same issue. And yes, he can join
Can you elaborate? The way I'm reading that sentence - you get the same issue but he can join? Are you able to connect to the server hosted on your computer, from your brothers computer? Be sure to use your LAN IP address for this.
If you can join from your brother's computer - there's something wrong with your port forwarding. If you can't join from his computer - there's at least something wrong with server computer itself. Most likely a firewall or antivirus.