Ionic4 FCM push notification issue

I am developing ionic4 app and I integrated push notification in-app using the following link. I just copy and paste the following link code to android app. I am getting a notification if I send notifications from firebase develop console when the app is open or minimized. but if I close or kill the app. I am not receiving a notification. I integrated push notification from the following link. https://capacitor.ionicframework.com/docs/guides/push-notifications-firebase/

if no one is is able to answer this issue, why did you people are active in this ionic support and what is use of this support page. if you can not able to respond code issue given capacitor by official site then what is use of this forum. no use. if can not fix or answer close ionic and support for native .