Very simple script that read all monitors of UptimeRobot and send a notification to ntfy.sh when a monitor goes down.
Copy the config.example.php
to config.inc.php
and put your UptimeRobot API KEY (read-only key) and your ntfy.sh topic (authentication not yet supported).
php run.php
Try the script and if it works you can run in cron every 5 minutes:
*/5 * * * * www-data cd /path/to/script && php run.php