I finally found the solution. this problem is caused both by a glitch in Windows 10 and this game not allowing the player to search for a game by specifying an IP address. In windows 10 you can change the priority of networks - there are settings for them but they don't apply. so the game just searches on whatever the default connection is. in my case it was VMware's virtual adapters. and that seems to be the only connection this game was looking on rather than looking on all of them. So I had to disable every other network connection - then it worked.

Temporary Solution:

Disable all other network connections - as you cant change the network priority in Windows 10 due to a bug.

What needs to be fixed:

Either make the game search every network or give players an option to search manually by IP address or both.

Yes, all players that are trying to connect are using the same version - v1.4.1f1 DRM Free.

Router is: Netgear DGND3700v2

Modem is: Huawei HG659

Port forwarding is configured correctly on both the modem and router.

I cannot confirm that the steam version is not working as I don't know anyone that has it - just trying to play a LAN game with my brother with DRM Free version. Firewalls are all correctly set for this game. I have tried with several computers both Windows and Linux couldn't get any of them working.

Does port forwarding even need to be configured for a LAN game? Also is there anyway that I can tell the client which IP to look for, rather than relying on the game to search for it?