EX8000 is slow and also can't get to admin login page

Bought an EX8000 extender yesterday and set it up to connect to and extend signal from my Apple AirPort Time Capsule 802.11ac router, which has been an extremely stable and reliable device. I'm not happy Apple has chosen to get out of the router business. There are just some weak spots on the second floor of the house, which make a mesh extender desireable.

Set the extender up using the "Nighthawk" app, which was glitchy, but I got through it, including a firmware update on the extender. After unplugging it and moving it futher away from the router as instructed, and after booting back up (takes 4-5 minutes), lights indicate good connection and 5GHz channel backhaul. Connecting through the extender, I cannot get better than 10.5Mbps from the extender. It is also difficult/impossible to get to the extender's admin loging page. Just hangs and eventually says it won't open, even using Firefox.

I reset the extender to factory setting and re-set it up, from now on using the browser interface. Did this three times. Same results each time, locating the extender in several different areas. Lights all say good signal, 5GHz backhaul. Speed coming out of it is <10.5Mbps down (~24 up). Turn the extender off, stand next to it and connect directly to router WiFi, measure 100, 150, 200+ Mbps down directly from router. Only once or twice have I been able to connect to the admin login on the EX8000 after initial setup, but when I did, status indicated strong connections and speed settings at max available on both signal frequencies. 

Surely the hardware is a dud, so I reset it back to factory, returned it and swapped out for another one. Set that one up with the exact same paltry results. It is worse than useless, not only not filling out the signal gaps, but but also creating new places where the signal is unusably slow. This is not what I intended to spend my day doing. "Easy setup," it says, "works with all routers," it says. The first clearly isn't true, and I fear the second isn't either. Anyone had any luck with this thing connecting to an Airport router?