What does NCAS disconnected mean in event logs?

Zyxel_Dean

Upon seeing this log "No response from NCAS over 15 seconds: NCAS disconnected", this indicates the AP has lost connection with Nebula Cloud Authentication Server (NCAS). 
When the connection is built again, the event logs will show "NCAS connected: 204 Server is alive".

Impact:

When the AP is disconnected from NCAS, no longer will cloud authentication be available to authorized clients. Instead, the authentication falls back to "NCAS disconnection behavior", which you can choose to allow all connections or limit the access under this scenario.

Direction: Configure > Access points > SSID Advanced settings > Captive portal advance setting > NCAS disconnect behavior

To check the connectivity, traceroute to s.nebula.zyxel.com. The servers are hosted on Amazon Web Services (AWS), you can trace the route and check if the route is available to pass through Amazon domains. 

Direction: Devices > choose the AP > Live tools: choose Traceroute and input "s.nebula.zyxel.com" > Run