Fallback mechanism when FCM service cannot reach Android device via ports 5228, 5229 and 5230?

[Ricardo De Lemos]

Hello,

Does FCM have a fallback mechanism to in the case where FCM cannot reach the Android device using ports 5228, 5229 and 5230?
I was reading the documentation for FCM and APN, and noticed that APN has a fallback mechanism where it uses TCP port 443 (on Wi-Fi only) if devices can't reach APNs on port 5223.

Does FCM have a similar fallback mechanism?
Does FCM try to reach the device (via mobile data) if it cannot reach it via WiFi?

To illustrate, some of the scenarios, commonly found on Enterprise:

1) The user is connected to his company WiFi (mobile data = OFF) and internet access is only possible via HTTP proxy through 443 or 80. Will FCM be tunneled over HTTP?

2) The user is connected to his company WiFi (mobile data = ON) and internet access, via WiFi, is only possible via HTTP proxy through 443 or 80. Will FCM be able to reach the user using the mobile data?

Thanks in Advance,
Ricardo

[Ricardo De Lemos]

Anyone from Firebase development that could provide feedback on this?