My friend can't join this specific server of mine, but can join others including ones I own. They can also join other public servers that aren't aternos. I've logged into their account from my computer and tried joining, but it doesn't work. Other friends of minecan join this server.
We have tried restarting the server multiple times, using the Dyn IP, changing the DNS server, and turning off the firewall. They have been able to join in the past, but suddenly they cannot.