If it does not work, don't throw random hate at this, comment on the bottom of the page (comments section) and talk it out there.

I appreciate it.

First of all, locate your Left 4 Dead 2 game on Steam.

Then Right-Click it, and select Properties
Like so:

And then you'll have this window in front of you:

Then you'll have to open the Launch Options window to input the needed command.

Then just write in the following code:
+clientport 27030

So that it looks like this:
Then Click the "OK" button.

That's that sorted!!
It should work now, but in case it doesn't, here's a list of ports that you can use instead of 27030.
The quality of this screenshot isn't the best as this was really hard to find.
(These are not random numbers, these are the official ports that L4D2 uses to connect to the Valve servers)

After further investigation, it seems that the fix might bug out the game sometimes when connecting to servers (although very rarely).
The solution to this bug is just to restart the game without the command, and restart again with the command. After that, should be good.

Give me your thoughts on this Fix and if it worked down below, and if it does then don't forget to like!!
Take care everyone, and give 'em hell!!