@JimRockford
You might be experiencing a different issue; for personalized assistance you can make a new Help topic and fill out the form with the exact symptoms you're seeing and your domain name.
Having anything specific to Let's Encrypt's name on your inbound firewall rules won't help anything, they're checking from multiple places, not from the same server that your client is connecting to.
For general information on how to have firewalling work with Let's Encrypt continuing to expand the locations that they're validating from, you can refer to this FAQ: