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.*