Ionic Native - Network

I need a way to know when the app goes OffLine - I used Network from Ionic native and placed

Network.onDisconnect().subscribe(() => {
  // firing an alert here

in the constructor of app.ts

I think I am supposed to see the alert if I deliberately put my device to OffLine state !!

The alert does not shows up.

My intention is to get the alert fired with ‘onDisconnect’ event

Any help ??