Apple – Support – Discussions – Airport Extreme blinking amber but WiFi ….  So according to Bob Timmons:

Try temporarily connecting an ethernet cable from your Mac to one of the LAN <-> ports on the AirPort Extreme.

Open AirPort Utility and click Manual Setup

Click on the word “Status” and a window will open with details about why the amber light is blinking. It may likely be a notice that a firmware update is available for your AirPort Extreme.

You can disconnect the ethernet cable once you have closed AirPort Utility.