Adding Airport Express to existing AirPort Extreme wireless network

Is it possible the ethernet between the buildings is not working properly?

Does the link light come on when you plug in ethernet?

https://support.apple.com/en-us/HT202434 See this for the link lights on the ethernet port.. note also you should be using the WAN not LAN port to configure this.

I would suggest if you are having issues.. take the Express inside and plug it into the extreme with a short patch cable.. run through the setup wizard and note carefully that it should be extend by ethernet NOT extend by wireless.

If it is not working correctly configure it manually.. this is perhaps the best way when thing go wrong with the automated wizard.

Open the airport in your airport utility..

On the Internet tab make sure it is set to DHCP.

On the wireless tab make sure it is set to create a wireless network. Fill in the SSID yourself to match the Extreme.

On the Network tab select off (bridge mode).

Update the Airport and then plug it in.. if it does not work then please post a few screenshots from the airport utility of the extreme setup as well as express so we can see what is happening.

Unplug the AirPort Express and move it very close to the AirPort Extreme

Connect a short spare Ethernet cable (that you know is working) from a LAN <--> port on the AirPort Extreme to the WAN "O" Port on the AirPort Express

Power up the Express and let it run for a minute or two

Hold in the reset button on the Express for 7-8 seconds and release

Allow a full minute for the AirPort Express to restart to a slow, blinking amber light status

On your Mac......

Click the WiFi menu at the top of the screen

Look for a listing of New AirPort Base Station

Click on AirPort Express below that listing

When you click on AirPort Express, that will start up the setup "wizard". The wizard will take a few seconds to analyze the network, then display a screen that looks similar to example below except that you will see your devices and network name pictured.

Type in a device name that you want to call the AirPort Express

Click Next

Watch the next screen very carefully to see that the Express is being set up to extend using Ethernet. This will confirm that the setup wizard has picked up the Ethernet connection between the AirPort Extreme and AirPort Express and that that correct settings will be applied to the AirPort Express.

When you see the message of Setup Complete, click Done......and you are now done with the setup for the AirPort Express.

Power off the AirPort Express and move it back to the remote location

Connect the Ethernet cable to the WAN "O" port on the Express

Power up the Express

If the long Ethernet cable is working correctly, so too will the AirPort Express be working correctly.

If the Express does not work correctly at the remote location, you have a problem with the Ethernet cable or Ethernet connectors......since you have already confirmed that the Express was set up correctly with the short Ethernet cable that you used for the initial configuration of the Express.