and followed it to the letter except a few of the directives are for Apache 2.2 - I am on 2.4 (at least I think that is what it didn't like - not a strong Apache guy just yet...)
Anyways, in Googling around, I see this was a thing a while ago, but several posts said it went away - well, it's back at least for me - I can't pull up my site from an iPhone now that I am using Let's Encrypt certificates.
Typically the problem in this case is a missing chain certificate. You need to use fullchain.pem (in Apache 2.4) or chain.pem (in Apache 2.2) to ensure that your site is serving the intermediate certificate that proves that Let’s Encrypt itself is a trusted CA. If you fail to do this, the normal symptom is that the site will work in some browsers and not in others.