Question: I want to expand my home network with the ASUS RT-N66U. I want several workstations in one part of the house to be wired, but the other workstations in another part of the house to be wireless, while at the same time, being able to access all the machines (e.g. printer) in the network via LAN. 

Obviously, connecting the LAN side of the main switch to the WAN socket of the wireless router doesn't do anything because this means that I won't be able to access anything that isn't connected to the wireless router. 

I've followed the instructions [here](https://superuser.com/questions/104043/extend-network-by-connecting-a-wireless-router-to-a-wired-router), but after doing so, I get an error message saying that the router isn't connected to WAN whenever I try to access something on the internet.

To sum up everything, here is my setup:
ASUS RT-N66U connected to a switch's LAN port via the ASUS router's WAN slot. The switch is connected to the modem

And here is my problem: Although I can use the internet, I am having trouble connecting to a printer that is wired up to the switch. How can I make it so that I can connect to the internet and access all other machines in my home network.

Answer: 
Looks pretty straightforward to me. Use the ASUS RT-N66U as the main router (default configuration) for your network by connecting its WAN port to your internet connection (I suppose Cable or DSL modem but was not specified). Then connect all of your switches to any of the RT-N66U LAN ports. If your switches don't have auto-detect ports you need to use the uplink port or a crossover cable. This configuration will create a single LAN composed of all your wired and wireless devices.

Here I assumed that you already had something like a wired router for all your devices and that you don't mind replacing it by the ASUS RT-N66U but it was not specified in your question.
I know this is an old thread but was an issue for me just recently. The way I got around it was that the ASUS router needs to be in Wireless router mode.

Connect the ASUS router's WAN port direct to your cable/fibre modem that was supplied. Do not connect this side to the switch as this will cause routing problems.

If like me the router is far away from your modem try using home plugs this will extend it's reach. I use Devolo and they seem to be very good.

Then once connected and you have an established internet connection connect one of the LAN ports to your switch. And hey presto. Should all be working. Took me a long while to figure that out! Hope this helps someone.