Resurrecting this thread from the past. It's now New Years Eve 2019. I used the camera for a massive family photo and the card won't read with any of my computers, but it still reads in the camera, so I am resorting to the WU-1a adapter (wipes dust off). I too had the same problem folks above said. I tried every iterations possible, but I eventually got it to work. I did do the 10-second reset a few times, so that may have aided too. The clincher was the airplane mode. Once I was able to connect to it in airplane mode, opened the Nikon WMU app on the iPhone and clicked the wifi signal in the top left corner. Once that icon turned to connected I was finally able to connect long enough to get 24 photos downloaded. It would die out at about 10% without the airplane mode. I'm hoping to upgrade soon to a phone from this decade (yay, the roaring 20s!). Good luck to you all.
Summary: Reset the adapter by holding down for 10 seconds until the light becomes orange. Turn the phone off. Turn on. Turn on airplane mode. Connect in Wifi settings. Connect in App. Start downloading.