Before I start, yes I port forwarded, yes I tried to make my IP static, and no, hamachi doesn't work (I don't want to use hamachi anyway). Yes, I did everything right. How do I know I did everything right? Because people were able to join for a day or so. Let me explain.
So I decided I wanted to find out how to make a server without hamachi that was hosted on my pc. After tons of time trying to work my way around Xfinitys crappy port forwarding system, I got it to work. I was using paper 1.15.2/1.16.1 and everything worked fine, my friends were able to join (through my public IP address, no hamachi). We screwed a bit with plugins and did a few other things. There was a problem though, and that was that 32-bit Java doesn't like more than 1gb of ram allocated to the server. After screwing around with some plugins, my friends left the server so I could resolve the issue since the lag was too great. I found the solution was to get 64-bit Java, so I did as intended. That's where it went downhill though... as soon as I installed 64-bit java, my friends could not join through my public IP. I tried using a separate computer on the same network, tried joining using both the private and public IP, to no avail (yes, I used a separate account if that makes any difference). The only way I could connect to the server was on my own computer using my private IP. Tried using basically every IP I could find in command prompts "ipconfig /all" command. Tried using hamachi, nothing. Tried going back to 32-bit Java (I used the same installation file that I had used to install 32-bit Java in the first place), nothing. Made my IP static, tried changing server properties such as server IP and port (yes changed the port forward too when I changed the port). Nothing. There's probably quite a few more things I tried doing that I can't remember or that were just dumb and desperate ideas. I don't know what happened and how it happened, but all I know is that when I reinstalled Java I couldn't join at all unless it was from my own machine that was running the server using its private address. I will say once more to emphasize that I used the same exact installer I first used to install Java (32-bit) on the machine when trying to revert back to the first Java instance I used. I've been troubleshooting this for a week now, and I've gotten so tired that I finally decided to turn over to the internet for help. If anyone out there has any idea why this is happening, PLEASE let me know, I just want to play minecraft with my friends, not suffer over computer issues. Thanks, Macker
Are you port forwarding 25565? Also, enter the inbound port as 25565 and outbound port to 25566. Leave the IP blank in the server.properties file. Finally, make sure you are using your IPv4 address. If it is not a firewall issue then im not sure what else to say :/
I ran into this issue a few times, and turned out it was my router not liking me changing settings after the server was offline which then caused the portfowarding to get permanently messed up until i factory-reset the router. After that I reapplied all my previous settings, then the problem was fixed.
Since you tried disabling firewall, the other thing you could do is reset your router. Check your router's manual to see how you can reset it to factory settings. If that doesn't resolve anything, then check your programs list against this link, to make sure that something isn't blocking the ports Minecraft is using. Beware that malware could have also caused this, so just to make sure run a malware scan.