Hello chwhite,
The MX450 can work as a firewall depending on how you are connecting it to the network. You have two options in the Addressing and VLANs section ( Routed or Passthrough). Could you please mention which option you have?
Additionally, all the MXs need an internet connection to talk to the cloud and be managed on the dashboard. The MX will use the WAN interface to get an IP address to reach out to the cloud. You should ensure that the WAN interface is connected to the internet.
After that the MX can act as a firewall from all the traffic that is traversing the device, could be from the LAN side to WAN or inter-VLAN traffic.
To help you in this one, we need to bring the MX online first.After that, we can continue talking on how to setup the firewall 🙂 .