If you use the Steam OnlineSubsystem then you need two different accounts on two different PC’s since you can only login to Steam in one Instance per PC.
Port forwarding is not necessary if you are using Steam since it will help with the NAT hole punching, but if you are simply directly connecting to an IP then the Host / Server needs proper Port Forwarding setup.
If you want to use Steam then
[OnlineSubsystem]
DefaultPlatformService=NULL
is wrong. It should be
[OnlineSubsystem]
DefaultPlatformService=Steam