When trying to "apply" changes, get a "400 Bad Request"

Be sure to clear out all browser caches before entering the routers web page.

Try a different browser, IE11, Firefox or Opera. 

Try a factory reset and setup from scratch. Don't load any saved config file from back up.

A complete pull of the power adapters for a period of time after the factory reset then walk thru the setup wizard and setup from scratch with a wired PC and web browser. https://kb.netgear.com/22697/How-do-I-install-my-NETGEAR-router-using-the-router-web-interface
Recommend setting the default DHCP IP address pool range to the following after applying and a factory reset: 172.16.#.100 to 122.16.#.200.
https://kb.netgear.com/24089/How-do-I-specify-the-pool-of-IP-addresses-assigned-by-my-Nighthawk-router
https://kb.netgear.com/25722/How-do-I-reserve-an-IP-address-on-my-NETGEAR-router
I would power OFF the ISP modem for 1 minute. Factory reset the router and power it off. Power ON the ISP modem and let it sync. Then power ON the router and walk thru the setup wizard again using a wired PC and a web browser.
Press the back reset button for 15 seconds then release. 
https://kb.netgear.com/31486/How-do-I-reset-my-Orbi-system-to-factory-default-settings
Or use the ERASE button on the routers web page to factory reset the router:
https://kb.netgear.com/000062081/How-do-I-erase-the-configuration-settings-on-my-Orbi-WiFi-System