Test firewall rules without plugging it into production?

I'm breaking my head here guys.

Supposed to replace the company's existing firewall. Sounds easy enough right? Well I've clearly made a mistake somewhere in the configuration because no matter what I do, nothing gets through the firewall.

I can't keep taking our internal, public-facing services down for 10-15 minutes every day to see if the changes I made are working.

So here's what I was thinking.. can I connect the firewall in question to a router, which has a client connected, that I can then use to test the firewall through port scans, etc.?

I need to be able to keep existing WAN settings on the firewall, and I'm really confused about how this setup would look like, if at all possible.

Hope someone smart on here can help out.