Same issue here.

What's the target ip address your port forwarding rules points at?

The target IP Address is the one assigned to my PC by the router, the IPv4 address

But the net diag still says not forwarded. Can you please paste me the results of tracert client.voobly.com in Command Prompt? Thanks.

The result seems to be endless, more than 20 requests took place out of which many time outs and some give a result. I think it means the connection is not directly being established?

I just need the first five hops.

Here

Try changing the port to 27000 UDP, in both, the Voobly setings and your port forwarding rule.
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Did exactly as said, the results are the same with the port not being forwarded. It's now directing traffic from ports around 28000 instead of 55000 as the earlier diagnostics said

Non-DirectPlay game path is 'C:\Program Files (x86)\Microsoft Games\Age of Empires II'
Verifying 'C:\Program Files (x86)\Microsoft Games\Age of Empires II\language_x1.dll' exists...
Success!
Verifying 'C:\Program Files (x86)\Microsoft Games\Age of Empires II\age2_x1\age2_x1.exe' exists...
Success!
Voobly path is 'c:\program files (x86)\voobly'
Game version is '1.0C'
Verifying 'C:\Program Files (x86)\Microsoft Games\Age of Empires II\language_x1_p1.dll' exists...
Success!
Checking DirectPlay libraries...
Looking for dpwsockx.dll...
Looking for dplayx.dll...
'c:\windows\system32\dplayx.dll' size: 215552 bytes
DirectPlay Init Success!
Scanning installed DirectPlay games...
Age of Empires II
Age of Empires II - The Conquerors Expansion [found]
Game was found in DirectPlay registry!
DirectPlay play thinks the game is installed at 'C:\Program Files (x86)\Microsoft Games\Age of Empires II'
DirectPlay command line is 'Lobby'
DirectPlay play thinks the game exe is at 'C:\Program Files (x86)\Microsoft Games\Age of Empires II\Age2_X1\age2_x1.exe'
Verifying file exists... Success!
DirectPlay play thinks the language dll is at 'C:\Program Files (x86)\Microsoft Games\Age of Empires II\language_x1.dll'
Verifying file exists... Success!
Scanning 'c:\program files (x86)\voobly\gamedata\AOC\age2_x1.exe'
Success!

Everything looks fine. DirectPlay is configured and the game was found on your harddrive. If you still have problems try rejoining the lobby or restarting your computer.

Your screenshot says PORT TRIGGERING. You have no port forwarding or DNAT?

There is no direct option for port forwarding.

There exist several terms for this. Port forwarding, Destination Network Address Translation (DNAT), Virtual Host etc.

If you provide a screenshot of your router interface for each menu, I can help you locating this setting. Every router supports DNAT these days!

Looks like Virtual Server in NAT Forwarding is what we are looking for. Please create a new rule in Virtual Server and take some screenshots again!