Cannot connect on 1.16.4 Forge server

Hello! a friend of mine recently started a modded server, but when we tried to play on it some of our other friends weren't able to connect

The connection error is:

"io.netty.channel.AbstractChannel$AnnotatedConnectException: Connection timed out: no further information"
(The server didn't have any mods yet, because we were gonna test if everyone is able to join)

Software & Version is Forge 1.16.4 (35.1.37)

The server is whitelist only too, but I am sure we put the correct usernames

I use Tlauncher with my minecraft account and could connect just fine. My friends also use Tlauncher aswell but with a Tlauncher account instead

hehe yeah I really don't huh

But my friends need it since they don't have a Minecraft account.

Thank you for sharing the article though!