DoubleSb

The way port forwarding works is by forwarding an external (WAN) port to your local (LAN) IP. If you don't forward the port or have the local IP on a DMZ, then any direct connection will be dropped. Imagine if you had 4 computers on your router and one was running the Minecraft server. How would the router know which one is running it? You only have 1 external IP and if I connect to 1.2.3.4:25565, it will ask the destination router how to get to connect to that port. If that port isn't forwarded then it's dropped.

If you want to start fresh then delete the world folder and restart the server.