DIAGNOSIS The local network is functional, but the router's connection to Netgear's cloud service, which the app uses for remote management, has failed. This is a common firmware or account sync issue.
FIX First, refresh the app. Swipe down on the main screen or pull to refresh. If that fails, log out of the Nighthawk app completely. Then, log into the router's local web interface at routerlogin.net. Go to Advanced > Administration > Router Status. Ensure 'Remote Management' is enabled. Now log back into the app.
IF THAT DIDN'T WORK Deregister the router from your Netgear account. In the web interface, go to Advanced > Administration > Router Account. Click 'Deregister'. Then, use the Nighthawk app to set up the router again as if new, which will re-register it with the cloud.
HEADS UP The app's 'offline' status only affects remote access via the app. Your local internet and Wi-Fi are unaffected. You can always manage the router directly by connecting to its Wi-Fi and going to routerlogin.net.