Hi,
Even if i don’t know Captive portal, i think your configuration is not right…
Basically, if you want to use Zeroshell as your main router, all of your computers must have the zeroshell IP as default gateway, and zeroshell must have your internet box IP as default gateway. Do not use the DHCP from your internet box, this box has to be invisible for all the computers. (if not, all the trafic may go directly to the internet box without passing through zeroshell)
Usually, with a basic configuration like this, you don’t need to care about the routing table.