The 2nd gen Ring devices are going to want to smart include by default which HE does not support at this time. If you look at the top in the comments of my driver that @SoundersDude linked there are very brief instructions to include the 2nd gen extender.
To get the 2nd gen device to start classic inclusion you have to exclude it (if you've played with it at all it requires a fresh exclude). Do that by unplugging it, putting the hub in exclusion mode and pressing the pinhole button on the back of the extender. Once it is excluded successfully you plug it in. You should see it immediately start to blink the LED green. This means it went into smart inclusion. To steer it to classic inclusion you hold the front button down for about 3 seconds. It will pair after that.
