Glad you were able to narrow down the issue to the firewall!
You can have one “active” Roon Core at a time, but you are free to switch between Cores as often as you wish.
You can also have an unlimited number of Roon Remotes/Clients connected to the Core, the Roon Remotes expose their audio devices to the Core, so if you just want to add the functionality of Roon you can use a Roon Remote or Client to connect to the Core. Note, your Core still needs to be powered on and active on the network for the Roon Remotes/Clients to be able to connect to it.
This is the new price unfortunately. The reason as to why we made this change is extensively discussed in this thread: