Have you tried from an external network form the public internet with a command like nmap -Pn -p80,443 example.com?
Do you possibly have some Geo Blocking?
- Let's Encrypt uses Multi-Perspective Validation Improves Domain Validation Security - Let's Encrypt
- Let's Encrypt is adding two new remote perspectives for domain validation
- Multi-Perspective Validation & Geoblocking FAQ
- Unexpected renewal failures since April 2024? Please read this!
And some tools to help check in addition to @petercooperjr recommendations
Edit
One more thing all IP Addresses need to respond the same, that is including all IPv4 and IPv6 Addresses.