This is NOT an issue with your firewall, ISP, or ports.
If any of my friends or I move a certain distance away from our island, we get sent to the main menu with Host Unreachable error and cannot rejoin often for 20+ minutes.
The same thing happens if we get too close to another players base on this same island. Crash to menu, Host Unreachable.
This is very much an issue related to the world becoming bloated after extensive amounts of building and exploration.