As Gnomish said, GOG and Steam crossplay has been available for years. Unfortunately, Grim Dawn’s mutliplayer is person to person (p2p), so we can only do so much to assist in connectivity.
If you do not have full access to your router settings, we can’t do anything about that because there isn’t a central server through which you are connecting to other players.
As for playtest access, I don’t see how this is an expectation, to be honest. The game’s alpha and beta were Steam only, and playtests have always been as well.
It makes no sense for us to run essentially two separate playtests and double the work of generating each build. When 96% of our audience is on Steam, that is where we are going to run them.