2 PCs behind a "modem-router"... help!

Hi guys... here's my first post. But let me tell you, before I create this one, I've readed most of the topics without gettin' successed. Ok here we go...

I played this game many years ago (last time was 2012/2013 year) with my brother, each one of us with his own computer, connected behind a same router. I always hosted a dedicated server using the "server manager v2.9" (such a great tool!) connecting me and my brother and other players joined too with us to play coop... Followed all the steps that you published here and followed other tutorials too (this means open ports, disabling TCP/IPv6, setting a static IP, configuring ServerManager, ect). Never had a problem, never! . In that time, we both used Windows 7 ultimate 32 bits (or maybe 64 bits, can't remember) and the ISP provided us a modem-router (coaxial cable and wi-fi service) but the device was locked! so I can't managed ports unless I disabled NAT, in order to function like a normal modem and no wi-fi. Separately, connected this device to a router I buyed (TP-Link TL-R460 Cable/DLS) and connected to it both computers. Golden memories...

Now...

After 5 years, we want to play again, following all the mentioned steps and... nothing!
I have upgrade to a brand new computer and running Windows 10 pro x64 (fall creators update 1709). My brother still has the same PC, even Windows 7 ultimate x64. We changed to another ISP, which provide us another type of modem-router device (no need to use my old TP Link anymore). This device allows us connect both computers and routing, managing ports with no restrictions like the other one.

The point of all I mentioned, is the new problem: I can host and enter successfully in my own server created with Service manager 2.9 (or without it too)... but when I'm in (character select n' equipment screen or lobby) my brother can't!! it shows to him the "CD key already in use" message. Or vice versa: first he can enter to my server and then when I try to enter I can't, showin' the same message to me LOL...

What I (or we, in some cases) do:

-Use same retail (pirated... sorry) version of the game (H&D2 and H&D2: Sabre Squadron) and updated to 1.12 and cracked
-Keep my same static IP, disablin' obviosly IPv6 and reset TCP/IP stack
-Open and managed ports from my device and firewall (11000-11500 and 2332 UDP/TCP)
-disabled IP helper service (aplicación auxiliar IP)
-Changed several times the CD Key editing the reg entry; or using the Serial Key Changer tool
-Execute the game and all its instances with admin privileges, or compatibilities modes (Windows XP SP3 nor Windows 7)
-Try setting server manager 2.9 both local and internet
-Patchin' the Host file in both computers
-Applying the H&D2v4Fix-x64
-..........

Nothing works! Maybe there's something I missing n' I don't remember what it is, but I'm about to givin' up, seriously
So the only way we can see my server on the list is add it manually, typing my local IP and port (192.168.1.110:11001) but just one of us can enter... I assume other players maybe can join? I don't know...
So I hope all my explanations are useful to understand my problem and really hope you can help me to solve it. If you need some pics about any of my configs, just tell me.
Thank you so much in advice!! Really!

(P.D: sorry for my english)

Hello Snowman!
Let me say to you, my friend, don't bother to do such a thing, in my opinion. I'm tellin' this 'cause I install de GOG version (maybe just like the Steam version it doesn't support multiplayer it seems) and the problem still remains, and reinstalled the retail version again... and nothing.
One point: This present time I didn't installed the GameSpy Arcade stuff... I can't remember if I did in the past
Second point: Yesterday I used the BION server and client, but I don't know how to use it properly, just "successfully connected"... but what else should I do? I created an internet server in-game with the BION running, I could enter in my server and one ramdom player get in too... but I closed the server for test purposes...
It must be something!... some technical issue. The one and only adaptor I have is my ethernet, not VPN and other thing
You can't imagine how much I wish they launch an important update fixing all the technical and connections issues; or remake the game. I don't want to get the idea to leave this incomparable game in the past
Greetings!! Waiting for someone to come up with a solution... or mod or something!

Like NL says: CD key problem. Steam community confirms the same issue for the steam version and GOG apparently too. They all say changing the key could be a workaround... but not in my case.
I tried today in my 3º computer (Windows 7 ultimate x64), installing the game and just executing only the server manager 2.9 (adjusting all the configs and settings previously done). Try to enter to that server from my PC and my bro with his PC... we both get the same results: just one of us can enter, even in a third machine (connected to our same router too)
It's really frustrating, guys. I just wanted to maybe give the chance to other people to play the game with a low ping from my area. The good flavor of the game is the coop mode that it has. Many greetings and still keep high my hopes...!