Using socketio to connect Flutter and RASA

I am new to both. I succeed in using REST channel to communicate. but my needs require external intent triggers and the bot to reach out to the user without the user saying anything (e.g., reminders).

I don’t think it is possible with REST, or if it is, I don’t know how. I would like to avoid callback as I don’t want to add another server to the list. I’d like some help with suggestions or resources. I couldn’t find anything works for me.