Meraki should trust LetsEncrypt. Their CA is dual signed by someone else who I can't remember as well, so it really should be trusted.
Maybe try using SSL tester to make sure it isn't something dumb like a weak hash for weak cipher is being used.
From what I know of, Letsencrypt is trusted by all major Root CAs. So in principle it should be secure enough.
On the other hand, I remember to have read somewhere that many don't see Letsencrypt as a safe free encryption body any more, due to its ease in creating SSL certificates.
Before, you could trust that if someone uses HTTPS the website was regarded as safe/free of malicious software, because certificates weren't easy to get, and people trusted it. With Letsencrypt, it has become too easy for malicious entities to create websites, even using Letsencrypt.
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