New Airport extreme as a wireless access point on all wireless network

Ethernet is always the best performance option when you connect AirPorts together on a network. If it is not possible the run the Ethernet cable, then the next best performance option would be to use a pair of Ethernet over Powerline (EoP) adapters to send the Ethernet signal over the existing AC wiring in the home.

In other words, the theory with these devices is that you already have the wiring in place, you just need the adapters. Any computer / electronics superstore will have a selection of EoP adapters from which to choose.

The last choice in terms of performance to extend a network would be a wireless connection between the AirPort routers. Unfortunately, even if you have a line-of-sight relationship between the AirPorts, about half of the potential speed on the network will be lost using this type of setup.

If the wireless signal must pass through several walls, then each wall will slow the signal speed even more. Much worse would be a condition where the wireless signal must pass through a ceiling.....since a typical ceiling is on average, about 3-4 times more dense than an average wall.

Please think about the options and let us know which one that you want to use, so we can provide more details on how to set things up.

Unfortunately, unless you do decide to run Ethernet cabling, things are always a bit of a guessing game.

In other words, you won't know how well.....or, even if.....the powerline products will work well, until you try them out in your home. So, it would be good idea to make sure that you understand the store's return policies before you buy.

Here's an example. I used pair of EoP adapters in my home a few years ago to get an Ethernet signal to a remote part of the house. The adapters worked very well.

My neighbor wanted to do the same thing in his house, and the very same brand and model number of adapter did not work well at all in his home......even though the distance to the remote area was less than in my home. Yet, when he tried another brand of adapter, those products worked well for him.

Two adapters are needed. They usually come in pairs.

Connect one adapter to an AC wall socket near the Centurylink router and connect a short Ethernet cable from one of the LAN <--> Ethernet ports on the router to the powerline adapter.

Upstairs, connect the other powerline adapter to an AC wall jack and then connect an Ethernet cable from the adapter to the WAN "O" port on the AirPort Extreme.

Apple's setup "wizard" will automatically configure the AirPort Extreme for you with the correct settings. All that you need to do is assign a device name for the AirPort Extreme.

After you have the Ethernet connection from the upstairs powerline adapter connected to the WAN "O" port on the AirPort Extreme and the AirPort has been powered up for a minute.......

Click the WiFi menu at the top of the Mac's screen

Click on AirPort Extreme under the heading of New AirPort Base Station

When you click on AirPort Extreme, that will get the setup "wizard" up and running. It will take a few seconds to suggest the correct setup, like the illustration below, except that you will see your devices pictured.

Type in a name that want to call the AirPort Extreme.......example......UpstairsAE

Click Next

Watch the screen to confirm that the AirPort Extreme is being set up to Extend using Ethernet. This confirms that the correct settings will be applied.The wizard will set up everything for you automatically. You don't have to worry about "Bridge Mode" or other things like that.

When you see the message of Setup Complete, click Done. And, you are done.

Sounds good. Once you have the powerline adapters installed.....and before you try to set up the AirPort Extreme......test the remote Ethernet connection at the upstairs powerline adapter by connecting your Mac directly to the adapter using an Ethernet cable, and turn off the WiFi on the Mac temporarily.

If you can get a good Internet connection that way, then you know that the Ethernet connection is working correctly over the AC wiring and the powerline adapters, so it should be an easy task to set up the AirPort Extreme.

Not a big deal, but If possible, connect the new Extreme back to the Centrylink router, if only because this would be considered better wiring practice.

If you connect the powerline adapter to the old AirPort Extreme.....and the old Extreme has a problem, then so too will your new Extreme likely have a problem as well......and it will be a bit tricky to figure out which Extreme really has the problem, since both of them will be acting up.

If you connect the powerline adapter to the Centurylink router....and the old Extreme has a problem, then the new Extreme will continue to operate normally.....(unless the Centurylink router is having a problem).....in which case both Extremes will have a problem as well.

Bottom line......connecting the new Extreme to the old Extreme would be known as a "daisy chain". Connecting both the new Extreme and old Extreme back to the Centurylink router would be known as a "home run". Home run is always the better option if you have a choice.

However, I noticed the light on the AE upstairs was flashing amber

Did you check to see why the amber light was flashing? If a new firmware update was available for the AirPort Extreme....that would be one reason for the amber light to be flashing.....to notify you of that fact.

Do I need to disable the wifi/router function of the Centurylink and just use it as a modem?

No