I did create a GCM account for my app to receive push notifications, now there is a new update which is mandatory for the users to download otherwise they will loos a lot of app functionalities due to server side issues.
So, when trying to send the users a notification to inform them about how important this update is, I couldn't find any way to send notifications in Google console like in Firebase console if I'm using FCM:
and upon searching, I found some tools like pushWatch, but there is no tool that can handle this huge amount of device ids, also it whole fails if there is at least one single device is already updated and can't receive it.
p.s. the update contains a new server APIkey.
There is currently no option to send messages to all your users using the FCM REST API without specifying a target (registration tokens). The option to send a message to all users is only available through the Firebase Notifications Console.
So from the comments, if all you need is a way to send messages without an App Server, you could simply send a request using Postman or cURL.