That isn't how this works.
Why on earth would you have two routers? Why would you need a router outside of the firewall?
The one and only router is behind the firewall, not in front of it.
Never use a DMZ, it is extremely ill advised. Plus a switch doesn't impact the network in any capacity, they do not have addresses, only the devices attached to them.
Firewall - Router with port forwarding - switches if needed - devices.
Just download pfSense and play with it, you can do all of this with one re-purposed device.