Rocket.Chat GCM notifications not showing in LineageOS 16 I am running a Rocket.Chat server of my own and everything works fine with almost all mobile device clients (official 3.5.0) using it. So GCM and gateway (gateway.rocket.chat) work as should and notifications are showing correctly.

Except one phone that has LineageOS 16 flashed is not showing Rocket.Chat notifications although every other aspect do work with the app.

is this a known issue with LineageOS 16? If not how could I find out what is going wrong? So howto fix it or is it even possible?

I did try to search but did not find any relevant information about this issue yet.
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 correctly. 

If you want to use an app that uses push notification via Google you have to install the Google apps, for example by flashing one of the Open GApps packages - at least the "pico package".

Afterwards you will be able to retrieve push notifications.