How can I make the app to send a notification when the user connected to the internet after being disconnected even when the app is closed?
You can use "@react-native-community/netinfo" library to check whether the internet is connected or not and based on that you can configure the push notification using react-native-push-notification library.
Click here for the reference!
Well I will recommend to use the alerts to notify the user wether the internet is lost or back based on your requirement.