I think I could help you with the issue.
You need to go to Notifications > Advanced and there you will find dual app
I'm not sure that the path will be the same for your device but for the record I'm using Samsung Galaxy S7 with Android 8.0.0 on it.
I assume you mean OxygenOS 9.0.8, based on Android 9 Pie.
To do this, you can go to Settings > Apps and Notifications > See all apps
Scroll down the list to find your app and select it from the list.
Choose which notification channel you would like to mute notifications from.
Tap Behavior and select your desired behavior.
- 'App & Notification'
- 'Yahoo Mail'
- Tap on the name of each type, such as 'Mail from people', 'Reminder' etc
- Under 'Behavior', select 'Show silently'
You have to repeat the last two steps for each type.
Typical messaging apps like Rocket.Chat use Firebase Cloud Messaging (FCM formerly known as GCM) for retrieving notifications of new messages from the server.
An Android side FCM required the Google Play Services app.
However LineageOS does not include any Google apps by default. Therefore all apps that use push notifications via Google/FCM will not work ...