Wireless uplink for Unifi AP

This used to work, but suddenly it broke. Here are some tips and tricks, the main thing is that you can’t uplink from a Unifi Enterprise Pro to a regular Unifi anymore (as of 2.3.6) and you need to be set to HT20 (20MHz channels in 2.4Ghz) for this to work:
Here are some valuable tips and tricks from http://thecyberfusion.com/2013/04/unifi-ap-unifi-ap-lr-tips-for-successful-wireless-uplink/

Make sure to upgrade the firmware on all of the access points on your network before starting to uplink. This will ensure that all of the access points are running the same firmware and that any bugs in the previous firmware are overwritten.
You will need to first set up the intended wireless uplinked access point with a hard wired Ethernet cable. Don’t try to set up the UniFi AP wirelessly. Once it is set up to your network, then perform the wireless uplink.
As of the current firmware, 2.3.9, the UniFi AP Pro will not wireless uplink to the regular or long range UniFi access point. because on the UniPro as of 2.3.9, it uses 5GHz for the uplink for more performance and the regular UniFi is 2.4Ghz only 🙁 See https://community.ubnt.com/t5/UniFi/Unifi-AP-will-not-connect-via-wireless-uplink/td-p/386405
Do not try to wireless uplink more than 3 access points to 1 hard wired access point. In my own experience, more than 3 feels a little unstable.
Before going wireless, it is a good idea to place the hard wired access point and the intended wireless access point on the same channel. For example, if you have an access point in your office and you want to wireless uplink an access point in the kitchen to the office access point, place them both on channel 1 before going wireless.
You will want to keep the radio to HT20 not HT40. The access points will not wireless uplink in the HT40 setting. (see image)
Make sure the compatibility of the access points is identical. For example, if the hard wired access point is set to 11n only, make sure your intended wireless uplink access point is set to 11n only as well.
You cannot wireless uplink to a wireless uplinked access point. The access point with which you are uplinking to must be wired to the network.
If all else fails, try resetting the intended access points. Use a paper clip to hold the reset button down for 30 seconds. Once it starts to blink, unplug the access point from the network and the power. Then start it back up…all the settings should now be back to default.

And to review at 2.4Ghz, if you use ht40, you are really vulnerable to interference. Probably want ht20 for 2.4 and ht40 for 5GHz.
I had to reset and adopt so if you are going to blow away a unifi controller (which lives as a Java app on a machine), you need to save a config frequently and have it available for restore. See http://gregsowell.com/?p=3201.
Here is Unifi’s not super intuitive method for making an “island AP” connect via wireless uplink to an uplink AP:
To enable wireless uplink:

Adopt all APs through wire first (using Ethernet cable). In other words, adopt both uplink and island APs.
Put the island AP to the intended location and connect its power. This means connect power adapter POE port to the island AP, but leave power adapter LAN port empty.
After the island AP is up, on the controller, wait until it becomes “Heartbeat Missed” and then “Disconnected” or “Isolated” state (takes about 6+ minutes). It will _not_ service any configured WLANs at this moment.
Go to AP dialog->Configure->Wireless Uplink, select the uplink AP of your choice (click on “Find more” if no uplink AP is shown)
The controller establishes wireless uplink between the selected uplink AP and the island AP. The island AP is now wireless connected and serving.

Technical details – Isolated AP

A new status, Isolated, is introduced. When the AP is unable to reach the gateway, it goes into Isolated state. In this state,

all servicing WLANs are disabled (if we cannot reach the gateway, wireless clients won’t either)
has different LED pattern – steady green (managed) with occasional dims
AP will send out beacon over the air and can be found by nearby APs
Only the wired APs under the same controller can establish a downlink to this isolated AP
by default, wired APs don’t go off-channel to look for isolated APs. “Find more” triggers wired APs to do so. And after wireless uplink is set up, the isolated AP will always find and follow the same channel use by its uplink AP