AX1800 4 Can not join network "NETGEAR_EXT"

What router do you have? 

What extender do you have? (hint: ax1800 is a speed, not a model)

go to netgear.com/support/  type in your model number. it'll take you to guides, manuals, firmware, etc. 

Are you setting up via wps or using the installation assistant? 

Have you tried hardwiring in for setup? It tends to go faster/smoother than doing over wireless or the app.