Well, you just told us:
That Argon is your "device already on the network", since it's the one creating the network in the first place. A gateway should be the one creating the network, and you can add devices from there on out.
You should be able to scan the Argon during the setup process, and go from there.
Updating them to the latest version using the CLI is also an option (that's considerably quicker than the app). There are also some new Mesh commands available (in Beta):
Mesh and USB commands in the CLI