Firebase custom Notifications not working when app is terminated

My Android app, written in Java, is facing issues with Firebase custom notifications. The notifications do not work when the app is terminated. This issue occurs with both data messages and notification messages.

Ideal Skills:
- Proficient in Java and Android development
- Extensive experience with Firebase
- Familiarity with handling Android app notifications
- Able to troubleshoot and fix issues efficiently

Your task is to identify the root cause of the problem and implement a solution to ensure notifications are received even when the app is not running.