I'm going to use SchildiChat Android version from github.com , and use in a local network so I can't connect to internet and get notification is very important . I can't use from Firebase to push notification . I found some open source library bu I going to solve this problem in client.
Follow this link : https://github.com/SchildiChat/SchildiChat-android/blob/sc/docs/notifications.md#element-notification-implementations for handling notification in local server,
There are two ways for solve your soluion
1 - Foreground sync mode (Gplay and F-Droid)
2- F-Droid background Mode (using AlarmMaager)