Pro WiFi Extender setup looping - Solid white for 10s then flashing white, activation fails

I'm having trouble setting up my Pro WiFi Extender. During the activation process, the app keeps giving me an error saying "Sorry, this isn't working - We're having some trouble activating your Pro WiFi Extender".

Meanwhile, the extender's LED behavior is stuck in a loop: it stays solid white for about 10 seconds, and then starts flashing white again. I haven't moved it from the setup spot yet. What could be causing this boot/activation loop and how can I fix it?

It's the newest model without an internal battery backup, and my main gateway is not in Bridge Mode.