How can I connect an AirPort Express to an AirPort Extreme without getting a flashing amber light?

Are both the Airport Extreme and the Airport Express brand new?

Try removing the voip router [non-apple router]. What happens when 2 or more routers are connected on a network is that each router is trying to automatically assign ip address from the same pool.

So, turn off and remove the non-apple router. Connect the airport exrteme, and then the Airport Express in another room. Run the Airport utility, and run the Airport Extreme as the “Main” for an “extended” wireless network, and list the Airport Express as a “wireless extender”. This will automatically configure your wireless so that you can use wireless all through the house, get good range, and not have to run ethernet cables.

Apple routers do not extend well with non-apple routers. If you absolutely must have the non-apple router connected, you should turn off its ability to assign IP addresses.

Try this support article from Apple. Their support center actually helped me a great deal in setting up a tricky, wireless-to-ethernet network to connect my Xbox wirelessly.

Damn! Never answer a 1-lurve question!