Problem with basic networking...

I am new to networking in Unity and therefore tried this basic code from a Youtube tutorial. It works like a charm locally with two instances on the same computer. But as soon as I try online with 2 instances on 2 different computers (I even tried from my android phone as well), I get no reaction from the script.

The IP in the script is my external IP. Locally it works... as well as 127.0.0.1 ofc!

I think my problems has to do with the port (here I chose 25001). I tried a few different ones.

I also tried port forwarding on my router - opening port 25001 UDP (and TCP) to traffic.

Absolutely nothing works :-(

Now I don't know how to continue - this was my entry point as I'm not much of a coder and kinda learn as I go along.

Here's the code:

 using UnityEngine;
 using System.Collections;
 
 public class Menu : MonoBehaviour {
 
     public string IP = "212.45.114.171";
     public int Port = 25001;
 
     void OnGUI()
     {
         if (Network.peerType == NetworkPeerType.Disconnected)
         {
             if(GUI.Button(new Rect(100,100,200,50),"Start Client"))
             {
                 Network.Connect(IP,Port);
             }
             if(GUI.Button(new Rect(100,150,200,50),"Start Server"))
             {
                 Network.InitializeServer(10,Port);
             }
         }
         else {
             if(Network.peerType == NetworkPeerType.Client)
             {
                 GUI.Label(new Rect(100,100,200,50),"Client");
 
                 if(GUI.Button(new Rect(100,150,200,50),"Logout"))
                 {
                     Network.Disconnect(250);
                 }
             }
             if(Network.peerType == NetworkPeerType.Server)
             {
                 GUI.Label(new Rect(100,100,200,50),"Server");
                 GUI.Label(new Rect(100,150,200,50),"Connections: " + Network.connections.Length);
 
                 if(GUI.Button(new Rect(100,200,200,50),"Logout"))
                 {
                     Network.Disconnect(250);
                 }
             }
         }
     }
 }