Forged Alliance needs a direct connection between all players of the game. To resolve connection issues FAF now uses the ICE networking technique to establish connections via players. This is also used and fully supported for multiple players inside the same LAN.
Port forwarding is no longer necessary with ICE and won't help your connectivity issues.
From our current tests and recent diagnosis, we are pretty sure that the ICE adapter has no technical issues itself. However, not all operating systems behave the same (e.g. each Windows 10 installation can be different due to installed patches, installed software, configured network adapters etc.). These are problems that FAF cannot fix for you.
Here are some tips that might help you still:
Due to a very difficult and rare bug the ice adapters first connection attempts may fail. This occurs only on a small minority of installations. The bug is mitigated by the adapter forcing a relay (proxy) connection on the third connection attempt. This may cause your connection process to take up to 30 seconds.
In case you're still unable to connect / don't want to wait that long, you can manually tell the adapter to force a relay connection. To do this, head over to the FAF client settings (top left), go to the Forged Alliance tab and select "Force relay connection".
If you're experiencing high latencies this might be caused by being relayed to a lot of peers via the faforever.com server. You can confirm this by opening the ice adapter debug window (click the button in the info window), if it displays "relay" as local or remote candidate for any other player, you have a relay connection to them.
This means the adapter failed to establish a direct connection. (Also see Unable to connect / connecting takes ages section about this) This may be caused by your local network but also by your ISP (Internet Service Provider). Due to the limited amount of IPv4 addresses some carriers are employing so called "Carrier grade NATs / symmetrical NATs" where multiple customers get hidden behind one public IP address. In this case you will always get a relay connection, which should in most cases be fine, the only thing you could do about this would be contacting your ISP.
Another reason for this behavior could be your NAT (in your router) assigning packet traveling out a random port instead of the same port as used in the local network. You can try finding out if your router does this and if it does take a look at its configuration if you're able to set it to mapping ports "static"ally.
In case you encounter frequent connection issues after the game has already started, please first of all verify your internet connection is stable. To do this use a tool like ping.canbeuseful.com or the ping tool in a terminal (ping faforever.com -t on Windows, ping faforever.com on macOS/Linux) to monitor your connectivity and latency while playing the game.
If you see issues in this test, please refer to your ISP and test/investigate your private network for technical issues. Try reducing network load on your and other devices, use an Ethernet cable instead of WiFi, reduce WiFi traffic, switch the WiFi channel, remove network switch, test on another device, ...
It's still not working!
This used to be an issue in the past but is resolved now. Playing with multiple computers behind the same router should work without any additional configuration