I've been trying for 3 hours... I keep getting the error: 

io.netty.channel.abstractchannel$annotatedconnectexception

I've seen everywhere on this site, youtube and other blogs that this is often caused by a java version problem, or a firewall problem. I've tried them all, I have java 16 and I tried allowing the firewall to all the java (and OpenJDK) and minecraft (including the launcher). I've also tried turning off the firewall from both ends (from my pc and the server host's pc).

We usually used hamachi to host servers but since the update, it doesn't work, so the host got into his router settings and used port-fowarding. We've went through every step very carefully, making sure everything was good. Still nothing... We tried every ip we could see : his own private ip, typing in 0.0.0.0, changing the minecraft server ip in the properties. I tried connecting to his public and private ip, with and without the :25565...

We then tried to change the java executable in the minecraft launcher, making sure we were using the most up to date one (java 16).

I tried doing the ipconfig/flushdns... ipconfig/release... etc. etc. as an admin, still not working

At this point I'm getting pretty bummed out and I'm wondering why such a popular and massive game as Minecraft has such I complicated online system ! Why does it need to be like this and why did nothing I tried work, when it seems to be working for other people.