Notify when somebody new connects to WiFi?

The WAC120 listed in  your footer isn't a typical router. That's where you'f expect to monitor things as they join the network. After all, the router is in charge. So it may not be possible to find any wireless extender that will alert you to new clients.

You may be able to persuade the Netgear desktop genie to play ball. .

genie | Product | Support | NETGEAR

This has a setting on the Network map page to show a popup when new devices connect to the network.

You have posted your message in the section of this community given over to General WiFi Routers (Non-Nighthawk). (This is easily done, given Netgear's complicated community structure.)

Many questions apply to different types of device, so there may be useful responses here, but you might get more help, and find earlier questions and answers specific to your device, in the appropriate section for your hardware. That's probably here:

WiFi Range Extenders & Nighthawk Mesh - NETGEAR Communities

I will ask the Netgear moderator to move your message.

In the meantime you could visit the support pages:

Support | NETGEAR

Feed in your model number and check the documentation for your hardware.

You may have done this already. I can't tell from your message.

I mention it because Netgear stopped supplying printed manuals and CD versions some years ago and people sometimes miss the downloads.