Does this rely on FCM or anything blocked by China

[bilogic]

1. I'm looking for a group chat app with notifications that work in China
2. E.g. Mattermost, rocket chat were dropped from my list because they relied on FCM
3. Does PUSHER rely on FCM?
4. If PUSHER is blocked in China, can it be replaced with a self hosted option?
5. By notification, I specifically mean messager is installed as a PWA and currently closed, but shows a notification icon on B's Android/iOS when I send B a text who might be in China

Thank you