Minecraft Modded Server - Connection timeout Mac

Hello guys,

I've reduced and added mods from the ''Revelation'' modpack from FTB (1.12). I've struggled, but managed to make my own server with my modpack and enable it with enough RAM using the common ''java -Xmx5120M -Xms1024M -jar forge-1.12.2-14.23.5.2847-universal.jar nogui pause'' in the terminal.

However, my friend who is trying to log in (I'm in the UK and my friend is in Switzerland) to my server using my original IP (router IP) struggles to connects and only manages to connect after at least 5-6 tries.

The 5-6 times before he manages to connect, I have these two errors in the server terminal: 

[Server thread/INFO] [minecraft/NetHandlerPlayServer]: *PlayerName* lost connection: Timed out 

OR 

[Netty Server IO #11/ERROR] [FML]: NetworkDispatcher exception

io.netty.handler.timeout.ReadTimeoutException: null

This is the list of things that I've tried to do, that didn't change anything:

1) changing amount of RAM allocated (add / decrease) (on my computer and my friend's computer).

2) get rid of mods (both computer + server).

3) stop web security (avast, etc..).

4) stop firewall in my router.

5) change the IP to which my friend connect to (tried different ones).

6) get rid of ''online-mod'' in ''server.properties'' in server folder.

We have:

- same Minecraft version.

- same mod version.

- close RAM allocation to game.

- same launcher.

- same java.

Finally, my question will be how to stop this from happening?