A Friend of mine can't connect

So, recently i have been playing modded minecraft with my friend using aternos's server, and someone else wanted to join, but i don't know why he can't join in the server , he could join any other server but not mine , everytime he joins he recieve this message

"io.netty.channel.AbstractChannel$AnnotatedConnectException: Connection timed out: no further information:"

Anyone know how to help?

The Problem still occurs though , already double checked everything and yet he still recieves the same message