Hey!
I found a solution and a fix that 100% works. What I did was I login to my router and looks for something called DMZ and there's a box you have to fill in, inside of that text box, type in your IP address. After that, I checked the NAT type and it was opened. Next part is optional if you're not on windows, go to the windows 10 search bar and type in Windows Firewall. Once the firewall pop up, reset your settings. Next, check your NAT type. It should both be opened. This took me a whole month to figure out. I still got the same problem but after a few weeks I found out that the world was a bit broken and I told him to rejoin and the problem was resolved and I've never get the "Unable to connect to world" error ever again.