Push Server Plugin Error "Remote-Server-Not-Found"

Hello, i am a beginner who is trying to develop a chat application for the back-end side. Now i am trying to make a push notification if the user was offline. I already going through the internet and found out the “Push Server” and “Push Notification” plugins. Firstly, i tried the Push Server plugins but i was getting an error when i tried to sent IQ Staza to “push.example.com”. I got these error below:

<iq type="error" id="4M8wo-xx" to="user@my-service-name/mobile" from="push.example.com">
<command xmlns="http://jabber.org/protocol/commands" action="execute" node="register-push-fcm">
<x xmlns="jabber:x:data" type="submit">
<field var="token">
<field var="device-id">
<error code="404" type="cancel"><remote-server-not-found xmlns="urn:ietf:params:xml:ns:xmpp-stanzas"/>

And when i was looking for the answer of my error code, i found this topic and tried to follow the step. But still cannot manage to solve the error. Am i missing something? Thank you so much