Requirement is to implement point to point communication via FCM notifications between two Android devices without using Google Play Service preferably with or without using any external server. DeviceA(Android 8.0+) will have google play service and DeviceB(Android 8.0+) will not. They will need to exchange text(not SMS) data between them via notification. Device B may have to use a IOT like notification protocol called MQTT.
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Why use FCM? you can use MQTT directly. all you have to do is setup MQTT server and then devices can speak to each other via sending messages on topics I can help you with this Message me and we can get started