Have you kept port 80 open? It's still looking like nothing can connect to me:
If you're looking in logs, whenever you ask Let's Encrypt to validate your challenge (whether directly, or Let's Debug asking the Let's Encrypt staging system), you should be seeing at least 5 requests coming in. If you're not seeing them, then something is blocking them. If it's not on the server you're looking at, maybe it's a firewall or network further upstream.
You mention that you do intentionally block some IPs, so here's some information on blocking in general and how Let's Encrypt needs to check from multiple places around the world, in case that's helpful to your understanding of how the process works: