Netgear App & AX1800 Extender

Recently installed AX1800 extender automatically picks up the current (Netgear Nighthawk) cable router wifi network name (SSID).  Using the Nighthawk App I get the "Extender Not Set Up".  Following app prompts I get directions to join NETGEAR_EXT.  The next prompt says "Choose Network" - it's not listed.  However, two listings of the cable modem network show up - one with "Available via Nighthawk" added to the name and one without.  Both appear to work.  The next prompt is "wifi connected" then recycle back to "Choose Network".  This renders the app all but useless - How to I properly register the Extender to the app and which network name should be used?