Can't get EX3700/AC750 to extend WiFi

I just bought the AC750 wifi extender and spent over 5 hours trying to set it up. First I attempted to use the WPS method, but the WPS LED on the extender never connected and never really made a connection to the router, failing step 2 of the manual's instructions. So I moved on to the Web Browser of the booklet and tried to do that. After connecting to it with my laptop, only Internet Explorer would allow the provided URL in the booklet to connect, and I could then finally start the setup process. But here's where it got strange. Despite having followed all the setup instructions; selecting "WiFi Range Extender", choosing my router's SSID for both 2.4 and 5GHz, putting in the password, and starting the setup, the connection to the 5GHz band would ALWAYS fail, so I'd skip it to at least get the 2.4GHz set up. After setting up the extender's SSID and password, I'd hit finish, but anything connected to the extender would show "no internet" when connected. My phone redirected me to the setup page again, telling me "You've created an access point, but there's no connection". At no point did I try to set up an access point, but that's all it seems capable of doing despite my best efforts. Why won't it connect to my router?