[MovedGoalPosts]

I think you are trying to achieve something similar to my office's network. This is setup as follows:

Router WAN port - ISP allocated IP
<wired to>
Firewall / Spam Filter - ISP allocated IP (runs in transparent bridge mode)
<wired to>
Server - external facing network port - ISP allocated IP

Server - internal facing network port - Internal allocated IP from server's DHCP
<wired to>
switches
<wired to>
Workstations - internal allocated IPs from server

Note that the server therefore has an internet facing ISP provided IP address allocated in the same range as the firewall / spam filter and router. This allows the server to operate things like exchange and an external website for things like outlook's web access to which I can point the DNS records for website addresses. Certain ports had to be opened in the router's firewalls to allow traffic through to the webserver.
Our's office's server also acts as a firewall to the internal network, providing protection to that network from intruders and stuff. Thus only the website stuff can be seen externally. I suspect you could use another router instead of the server to fulfil a similar function, using this second router's WAN port with an ISP allocated IP addy, and then the router's DHCP to do the internal stuff.