Improved Bootstrap example scripts for Unity Netcode for GameObjects, see: https://forum.unity.com/threads/testing-netcode-through-the-internet.1209205/#post-8456006

BootstrapManager.cs

using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.NetworkInformation;
using System.Net.Sockets;
using Unity.Netcode.Transports.UTP;
using UnityEngine;

namespace Unity.Netcode.Samples
{
	/// <summary>
	/// Class to display helper buttons and status labels on the GUI, as well as buttons to start host/client/server.
	/// Once a connection has been established to the server, the local player can be teleported to random positions via a GUI button.
	/// </summary>
	public class BootstrapManager : MonoBehaviour
	{
		private string _hostIpString = "<enter host IP or domain>";
		private string _portString = "7777";
		private ushort _port;
		private string _resolvedHostIp = "";

		public static string[] GetLocalIPv4(NetworkInterfaceType _type)
		{
			var ipAddrList = new List<string>();
			foreach (var item in NetworkInterface.GetAllNetworkInterfaces())
			{
				if (item.NetworkInterfaceType == _type && item.OperationalStatus == OperationalStatus.Up)
				{
					foreach (var ip in item.GetIPProperties().UnicastAddresses)
					{
						if (ip.Address.AddressFamily == AddressFamily.InterNetwork)
							ipAddrList.Add(ip.Address.ToString());
					}
				}
			}
			return ipAddrList.ToArray();
		}

		public static IPAddress[] ResolveHostname(string hostName, bool ip6Wanted = false)
		{
			// is it already an IP?
			if (IPAddress.TryParse(hostName, out var outIpAddress))
				return new[] { outIpAddress };

			try
			{
				var addresslist = Dns.GetHostAddresses(hostName);
				if (addresslist == null || addresslist.Length == 0)
					return new IPAddress[0];

				if (ip6Wanted)
					return addresslist.Where(o => o.AddressFamily == AddressFamily.InterNetworkV6).ToArray();

				return addresslist.Where(o => o.AddressFamily == AddressFamily.InterNetwork).ToArray();
			}
			catch {}

			return null;
		}

		private void OnGUI()
		{
			GUILayout.BeginArea(new Rect(10, 10, 300, 300));

			var networkManager = NetworkManager.Singleton;
			var transport = networkManager.GetComponent<UnityTransport>();

			_hostIpString = GUILayout.TextField(_hostIpString);
			_portString = GUILayout.TextField(_portString);
			var localIP = GetLocalIPv4(NetworkInterfaceType.Ethernet)?.FirstOrDefault();

			if (!networkManager.IsClient && !networkManager.IsServer)
			{
				if (GUILayout.Button("Client"))
				{
					_resolvedHostIp = ResolveHostname(_hostIpString)?.FirstOrDefault()?.ToString();
					ushort.TryParse(_portString, out _port);
					print($"Connecting to: {_resolvedHostIp}:{_port}");
					transport.SetConnectionData(_resolvedHostIp, _port);
					networkManager.StartClient();
				}

				GUILayout.Space(40f);

				var startHost = GUILayout.Button("Host");
				var startServer = GUILayout.Button("Server");

				if (startHost || startServer)
				{
					localIP = GetLocalIPv4(NetworkInterfaceType.Ethernet).FirstOrDefault();
					ushort.TryParse(_portString, out _port);
					print($"Serving on: {localIP}:{_port}");
					transport.SetConnectionData(localIP, _port, "0.0.0.0");

					if (startHost)
						networkManager.StartHost();
					if (startServer)
						networkManager.StartServer();
				}
			}
			else
			{
				var mode = networkManager.IsHost ? $"Host {localIP}:{_port}" :
					networkManager.IsServer ? $"Server {localIP}:{_port}" : $"Client connected to {_resolvedHostIp}:{_port}";
				GUILayout.Label($"Mode: {mode}");

				// "Random Teleport" button will only be shown to clients
				if (networkManager.IsClient)
				{
					if (GUILayout.Button("Random Teleport"))
					{
						if (networkManager.LocalClient != null)
						{
							// Get `BootstrapPlayer` component from the player's `PlayerObject`
							if (networkManager.LocalClient.PlayerObject.TryGetComponent(out BootstrapPlayer bootstrapPlayer))
							{
								// Invoke a `ServerRpc` from client-side to teleport player to a random position on the server-side
								bootstrapPlayer.RandomTeleportServerRpc();
							}
						}
					}
				}
			}

			GUILayout.EndArea();
		}
	}
}

BootstrapPlayer.cs

using UnityEngine;

namespace Unity.Netcode.Samples
{
	/// <summary>
	/// Component attached to the "Player Prefab" on the `NetworkManager`.
	/// </summary>
	public class BootstrapPlayer : NetworkBehaviour
	{
		private static Vector3 GetRandomPositionOnXYPlane() => new(Random.Range(-5f, 5f), Random.Range(-5f, 5f), 0f);

		private void Start()
		{
			GetComponent<MeshRenderer>().material.color = IsLocalPlayer ? Color.green : Color.red;

			if (IsOwner)
				RandomTeleportServerRpc();
		}

		/// <summary>
		/// If this method is invoked on the client instance of this player, it will invoke a `ServerRpc` on the server-side.
		/// If this method is invoked on the server instance of this player, it will teleport player to a random position.
		/// </summary>
		/// <remarks>
		/// Since a `NetworkTransform` component is attached to this player, and the authority on that component is set to "Server",
		/// this transform's position modification can only be performed on the server, where it will then be replicated down to all clients through `NetworkTransform`.
		/// </remarks>
		[ServerRpc]
		public void RandomTeleportServerRpc()
		{
			var oldPosition = transform.position;
			transform.position = GetRandomPositionOnXYPlane();
			var newPosition = transform.position;
			print(
				$"{nameof(RandomTeleportServerRpc)}() -> {nameof(OwnerClientId)}: {OwnerClientId} --- {nameof(oldPosition)}: {oldPosition} --- {nameof(newPosition)}: {newPosition}");
		}
	}
}