The steps I followed to get everything working with a custom private key:
- Go to each unit in your network and click the "factory reset" button once.
- Once everything is reset go to device #1 and press the "security" button once, then go to device #2 and press the security button once.
- Wait two minutes. (I think you have to pair 2 devices before you can add a third)
- To add the 3rd (or 4th or 5th) device. Click the "factory reset" button (why not?) and wait a few seconds.
- Press the security button on device #3, then press the security button on device #2. That should pair #3 to the first two.
The Netgear documentation says you need to hold these buttons down for two or 10 seconds. When we did that, it seemed to make sure that nothing happened whatsoever!