Battery-saving features can restrict background activity; turn this off in Settings > Battery .
APNs requires specific open ports to communicate. If you are on a restricted network (like a corporate or school Wi-Fi), ensure the following: How to Fix "Apple Push" Errors
TCP port 5223 must be open for devices to receive notifications. Battery-saving features can restrict background activity
Troubleshooting Apple Push Notification Service (APNs) Errors How to Fix "Apple Push" Errors
For iCloud Mail, ensure "Push" is enabled under Settings > Apps > Mail > Accounts > Fetch New Data . Network & Connection Requirements