Firebase deprecated their push notification api endpoint around 1st August 2019. For this reason your push notifications might have stopped working.

If this is the case, you can fix this issue easily on your server with 3 steps:

1) Open Server Side Code /controllers/ApiController.php

2) Go to line 135: curl_setopt( $ch, CURLOPT_URL, '' );

3) Replace '' with ''

Make sure the line of code looks like this:

  curl_setopt( $ch, CURLOPT_URL, '' );