You have to change your steam settings to attempt to use proton. Once you do this, steam will allow the games to play. Practically everything will work once you do this.

It’s been a long time since I actually used subnets, but IIRC you will need a physical interface for each network on the router regardless.

So let’s say you set up your /24 network into 2x /25’s, you will need an interface for the .0 network, and another for the .128 network

If you just have an interface for the switch, and another for the WAN connection, I don’t think subnetting will work for what you’re trying to do