Neither do we. Let's Encrypt, by design, validates from around the globe. As a result, your DNS servers (and yes, that should be plural) need to respond worldwide. So if you're using any kind of geoblocking, that's the first thing to stop doing.
But the DNSchecker result you linked illustrates the problem--your DNS server (singular, which is surely at least part of the problem) isn't accessible to most of the world. We can't help you fix that.