Ok, so here's the situation: Friend and myself both purchased DOS2DE from GoG. Downloaded the v3.6.54.8890b offline backup game installers and installed. Install worked fine for both of us (file verification was enabled). Both can run the game, however have been unable to establish a direct connect game between our two games (either friend to my online story game, or me to friend's).

In searching around for answers, saw suggestion to start the game twice on same PC and have those connect to each other, I have been able to do that and was able to connect, both via LAN and using the DirectConnect server ID code. So am concluding this is a connectivity over the Internet issue.

Here is where things get a little more complicated. Neither of us are able to setup any sort of port forwarding, or make any other router changes. In my case, am on a "body corporate"-provided Internet service, get a wi-fi connection (802.11n, 200 mbps link speed, so bandwidth shouldn't be an issue), but any sort of router config -- forget it. My friend has the more traditional wired network, router with ethernet connection into ISP-provided cable modem, but does not have the technical know-how for configuring port forwarding etc, so not an option either (plus their link speed is no-where near as good).

Is worth noting, I don't have GoG's Galaxy middleware installed (don't need it, don't want it, am perfectly comfortable with downloading the installers, doing my own patching etc), whereas my friend does have it. When select Story > Multiplayer > Online tab, we see different messages:
me: "Not connected to the online community servers"
friend: "Not connected to the galaxy servers"

Yeah, this is a bit of a long-read, but figure I'd provide details of what have tried so far and what constrains exist, so hopefully someone out there to make a suggestion on how can we get what looks like is going to be a great game connected together.

Had them try with both. Starting the game with the Galaxy middleware up (they see a list of public? games), and without Galaxy running (so starting using a shortcut link to the ...DefEd\bin\EoCApp.exe), that's when get that "Not connected..." message indicated above. Either way, we were unable to get a co-op game going between our two PCs (me to them, or them to me). At the time of posting (2 days ago), we both were on v3.6.54.8890b installed (downloaded from GoG, using the offline installers), so in case of friend, they didn't install via Galaxy, but rather same as myself, using the offline installer. This evening, my friend loaded up Galaxy and it updated their version to v3.6.58.1306. Checked the GoG website, there is no patch for v3.6.54.8890b to v3.6.58.1306 available for download at this time (as I understand it, there can be a few days delay before those are published, so that's complicated matters :D ).

We're both on Windows and only attempting to connect two players, so no OSX or local split screen use involved. In my case, yes. As mentioned above friend is not as technically proficient, however beyond possibly a web browser and yes, seems they do have a 3rd-party AV program (Nortons) installed, no excess programs running (both machines have 16GB ram, with virtual memory settings in place). Checked mine, can see two inbound rules (one for UDP and TCP traffic). Friend's firewall more difficult to check, am uncertain whether the rules were installed correctly, have been unable to cause the DOS2DE program to invoke a request to add the rules. EDIT: See comment below, have now been able to confirm firewall rules are in place No, don't have anyone else with the game whom would trust to attempt a direct connect with. Yup, no joy there either. Whenever I try, get the same "Not connected to the online community servers" message. Friend sees no servers/player names etc listed (but then, we're both in Australia, so non-peak times) however no error message either. Used the following for each test, LAN connections switched off / direct connections on. Passed direct connect server IDs to each other (me->friend and friend->me), both case got "connection failed" message. Yup, no mods installed / "gift bag"(?) options selected, trying to keep everything as clean/simple as possible for now. These are clean installs, nothing done beyond configuring for our screen res/gfx preferences. We're different parts of the country, so unable to test over actual LAN (I did test and was successful using two instances of the game on the same PC). Neither of us would be comfortable using VPN software to connect our networks together (and in all likelihood would be blocked by my end), and we don't use Discord. I tried this couple days ago on my machine, without any improvement. Had hesitated to have friend do same (years of telling them "thar be dragons in there" when discussing using the command prompt :D ). Anyway, got friend to try that tonight, and they did then get the firewall rule prompt and have been able to confirm the Inbound rules now exist. However, due to the fact we have now got two different versions, were still unable to direct connect :(

Apologies for the delay in response, "life stuff" :( Yes, saw that (thx for the heads up). We are both now on v3.6.58.1306, and ran through the same tests and unfortunately the results are the same. Still unable to establish a co-op game either me-to-friend or friend-to-me. Timezone difference might be problematic, however I've send you a friend invite so we figure out if there's a compatible time for all three of us separate from this public forum.

I am having this problem now as well, my friend on Steam and I can't continue a game we've been playing over the last several months. Both of us get the error message "Connection failed" when either of us tries to connect to the other. Tried all of the above methods to fix it. It ain't working.