As stated above you need a router. The boxes you see in retail stores that say "router" on them do not actually have one of the most important feature a true router has and that is the ability to "route" traffic between subnets.
The common devices you see called routers should be called gateways because they are designed to translate a single lan subnet to a single wan IP address. That really is all most people and most small businesses need.
You just have to search a bit harder or build your own.