This guide refers to this particular method of Online Play:
https://steamproxy.com/sharedfiles/filedetails/?id=753690007

Credit goes to the creator of the first, original guide in Russian:
https://steamproxy.com/sharedfiles/filedetails/?id=389374146

To Tussmorke from my discord server, who pointed us to the port in the first place.

And, of course, the person who created the program we'll be using in this guide - christoph:
https://sourceforge.net/projects/upnp-portmapper/

So, while using that particular online fix, you might have encountered the "This game session is no longer available" problem, where you would be unable to connect with a huge amount of players. Well, fear not - because there is an easy way to fix that.

In short, we have to port forward the UDP 23756 port to be able to connect with everyone. However, since port forwarding can be a pain in the ass, we'll be using this software, instead:

https://sourceforge.net/projects/upnp-portmapper/

The version that seems to work best as of right now is 2.1.1. In order to download that one, you can click on "Files" and get it from there.

(no viruses, I promise)

Once you download it, drag it to your desktop, or wherever your heart desires it to be. After that, just simply open it. You should see this window:
















Might seem a little confusing at first, but bear with me.
The first thing you need to do is click the "Connect" button, after which it should change to "Disconnect" instead, and click on the "Create" button in the "Port Mapping Presets" section


















Once you do that, a window should pop up. In it, you have to name the preset to whatever you like (I personally named it "FlatOut2", as you can see), after which you need add two ports (both TCP and UDP just in case) by using the "Add" button to the right of the window, and fill in the port information according to the image. Once that's done, click "Save".


















You should now see your preset in the right field of the program. Select it, and click "Use".


















And now, you're good to go! You should now see the ports you are forwarding in the top field of the program.


















Keep in mind, however, that you'll need to open the program and click "Connect" every time you want to play FlatOut 2 Online.

If you happen to be getting that message, you could try choosing a different UPnP library from the ones available in the program. You can do that by clicking on the "PortMapper Settings..." button under the "Connect" one and experimenting with different libraries in the dropdown menu.

And that's pretty much it! Stay unsafe on FlatOut's roads, and I hope that you all have fun.

Feel free to ask me any questions in the comments below, and I'll try my best to respond to them quickly.