[ubuntu] Basic firewall/content filtering set up (newbie?) topic

This is a bit of a newbie question, but... I have been using Linux for a long time, and I can't get a firewall box operational. Here is my configuration. What am I doing wrong?

GOAL: I am trying to implement the content filtering on all traffic, so ... filter all traffic before it goes to the wifi/router.

SET UP:
Cable Modem > firewall box > Linksys Wifi/Router

ADDRESSING for above:
192.168.0.x > eth0: 192.168.0.x, eth1: 192.168.1.x > 192.168.1.x

PROBLEM:
I can't get traffic to go through the firewall box to the Linksys router.

Is the addressing okay? (do all devices need to be on the same subnet?)

I have also consistently NOT been able to access the management console from across the network

Is there a known approach to static/dhcp settings? Perhaps DHCP for all interfaces is best?

Should I reboot all devices in series to reset them?

Thanks for your help.