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Hi @ElPancho, and welcome to the LE community forum
It seems that there is some GeoLocation blocking that is preventing some of the secondary validation systems from reaching your authoritative DNS systems.
Who's the DSP?
It looks like it's being self-hosted:
We have Godaddy as DSP that forward to ns and ns1 to our external IPs. Then our DNS (Windows Server) contains the ns and ns1 registries and all other A records.
I don't know if you need some more information to get a sense of what could be happening.
I believe you need improved availability and quality of the DNS Servers and be made geo region agonistic agnostic;
and improve quality of the DNSSEC as well.
The problem is with access to your DNS Server not your IIS web server.
Let's Encrypt queries your authoritive DNS servers looking for an A and/or AAAA record. These queries are failing from at least one of the secondary LE centers.
Do you have any firewalls or similar settings protecting your DNS Servers? That is the place to look. It looks like you run your own DNS servers.