iOS Local Push Notification using Cellular Data

We are working on a project that's required to be in a fully isolated network using VPN.
We would like to find if there's any way to sending and receiving Push notifications without using APNs servers.

We found a topic in Apple documentation "Local Push Connectivity" but it requires the iOS device to be using a specific SSID that requires WiFi to work as mentioned in Apple documentation:
"When a user’s iOS device joins a Wi-Fi network that has an SSID matching a configured SSID in your NEAppPushManager instance, the system launches your app extension and instantiates the NEAppPushProvider subclass within that app extension."
Source:
https://developer.apple.com/documentation/networkextension/local_push_connectivity

We also found a sample code from Apple's documentation, "Receiving Voice and Text Communications on a Local Network"
But the sample mentioned that it "Provide voice and text communication on a local network isolated from Apple Push Notification service by adopting Local Push Connectivity."
Source:
https://developer.apple.com/documentation/networkextension/local_push_connectivity/receiving_voice_and_text_communications_on_a_local_network

We want to build a project to send Push Notifications & VoIP Notifications in a local and isolated network using VPN and cellular data only.

The scope of work:
- Building an iOS app to receive push notification from a privet server in a isolated network and VPN.
- Build the backend to send push notification to the isolated iOS devices.

Please feel free to ask any additional questions to clarify the task more for you.

Thanks.