Nighthawk CAX80 won't save configuration data.

Something to try, reload the same FW version on to the modem with a wired PC. 

https://kb.netgear.com/23960/How-do-I-manually-update-the-firmware-on-my-NETGEAR-router

After it's back to ready, reset the modem and setup from scratch using the erase button:

https://kb.netgear.com/24233/How-do-I-erase-the-configuration-settings-on-my-Nighthawk-router

Be sure to clear out all browser caches.

Then walk thru the setup routine with same wired PC and web browser. Set up the system for your needs. 

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: 192.168.#.100 to 192.168.#.200.
https://kb.netgear.com/24089/How-do-I-specify-the-pool-of-IP-addresses-assigned-by-my-Nighthawk-router

Then save off a back up configuration to file soon after all has been setup:

https://kb.netgear.com/24231/How-do-I-back-up-the-router-configuration-settings-on-my-Nighthawk-router

Let us know if you can get this completed.