Push notification doesn't work on free edition

Summary
After lot of investigation, I am still not able to make work push notifications through https://push-test.mattermost.com (the default server).

Steps to reproduce
I am using Mattermost V.v6.2.0

Expected behavior
When I message or mention a user, I would like a notification to pop-up on my iPhone for example.

Observed behavior
The notification doesn’t come on the smartphone, it’s like it doesn’t work from the server.
I have already checked the logs, sync the time of the server, but I cannot help myself to understand if it’s work. I am not able to try another push notification server.

My Mattermost server is behind a NGinx reverse proxy.

Could someone help me to investigate deeper ?

Kind regards,