Pingdom helpfully offer a RSS feed of their monitoring nodes (https://www.pingdom.com/rss/probe_servers.xml) unfortunately, it’s messy, poorly constructed and not simple to extract a nice list of IPs out of so you can add them to your firewalls.
I created a simple script which extracts the IPs from this XML feed and present a list of IPs in both plain-text and HTML.
The output can easily be copied into the allow list in your firewall configuration and the list is updated real-time from Pingdom themselves!
Plain Text: https://scripts.dan.cloud/pingdom/probe_ips.php
HTML Output (in iframe):