two routers - RAX35 & R7850

If the main Netgear router is getting a proper WAN IP address, then that should be fine as is.

For the secondary router in AP mode, try allowing it to dynamically acquire an IP address.

Also make sure that you are connecting from the WAN port of the secondary router, to a LAN port on the main Netgear router.

The best way to narrow down any potential issues, is to check if the defaults at least work. If needed also consider doing a factory reset of the secondary router, then after doing a basic setup and giving it anew SSID and password (typically the same SSID and password as the main router, to allow for proper client directed roaming), put it in AP mode, ensuring that while in AP mode, an Ethernet cable runs from the WAN port of the secondary Netgear router, to the LAN port of the primary Netgear router.

When done that way, it should automatically handle everything else to become a properly transparent AP to any clients connected to it.

The nighthawk app can still connect to the secondary router if needed, though you will need to tap the home icon in the upper left hand corner of the app, and then tap the first entry that would list a router model, there it will show a list of all Netgear devices on your network, and you can select which one you want to control.

You can also switch using the network map.