So I see alot of people wanting to add another wireless router to their Network to get wifi in hard to reach places I thought I'd give a little blanket advise. This can be done, it can even work quite well, but if done incorrectly it will cause a dual NAT situation effectively segregating all the devices on the second wireless router from the rest of the network, making it difficult if not impossible for all the devices to communicate. 

In order to avoid that issue you need to bypass the Wan port on the second router. Simply said, don't plug the Ethernet line that will be feeding the second router into the internet port. Instead hook that line up to one of the ports you would normally plug your devices into. Then you need to log into the second router GUI and disable the built in DHCP server.

Now the second wireless router will act as a simple switch that just happens to have it's own wifi radio. All the devices on the network will communicate correctly and you won't have any port forwarding or dual NAT nightmares to deal with.

You can set both routers to the same wireless network name, though you want to be sure that they are operating on different channels so they aren't competing for the same airspace and it will make network handoffs for devices roaming around the network smoother as most devices won't see it as a network change but rather just changing to a stronger channel.

Edit for formatting and spelling