@MalmoTP
What's your extra air router - if you've bought 1-pack unit it should be R5 (the main unit with two ethernet ports).
Have you tried to use the WPS method (video) ?
Method 3: Via the WPS Button
1. Press the WPS button on your router, and then press the WPS button on the extender for 1 second within 2 minutes.
Note:
For more information about using WPS button on your host router, please refer to the router manufacturer’s user guide.
2. Wait until the Signal LED changes from from pulsing blue to solid blue, which indicates that the WPS connection is successful.
Note:
If the LED does not turn solid on, try again or refer to Method 1: Via the TP-Link Tether App or Method 2: Via a Web Browser.
3. If the extender connects to a dual band router, repeat steps 1 and 2 above to connect to the other band. Wait until the Signal LED turns solid blue, and the setup is done.