Hi,
According to your description ,you can try to Use Exchange server Health Check to find out if there are any configuration issues : https://microsoft.github.io/CSS-Exchange/Diagnostics/HealthChecker/
Also,you can try to change the value of IIS -> Sites -> Exchange Back End -> PushNotifications -> SSL Settings
as well as SSL Settings for the "bin" path within Exchange Back End -> PushNotifications.
Please feel free to let me know if any updates.