Can't configure AX40 as AP mode

Consumer routers, especially new borns, make bad APs in general. (Just my very personal point on that idea...).

Assuming you have used this process (applicable to all new RAX AX routers, too):

How do I change my Nighthawk router to AP mode after I’ve already run setup?

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1. Scroll down to Choose IP Address settings on this access point and select Get dynamically from existing router. The other existing router on the network assigns an IP address to this router while this router is in AP mode. 
   Note: The Enable fixed IP settings on this device (not recommended) setting requires advanced network experience. Only use this setting if you want to manually assign a specific IP address to the router while it is in AP mode.
2. Click Apply.

Note: After you click Apply, the AP device will change to a new IP assigned by your existing router. The NETGEAR genie web interface may no longer be available.  ...

So at the point #6 the router is not restarting, or not picking up an address form the DHCP server (checked the router [not the AX40] Attached Devices list/DHCP logs, ...?) , or isn't reachable on the IP address manually configured [have used one in your LAN TCP/IP subnetwork?], ... also try the WAN as well as any LAN port. Who knows...