I'm having a similar issue accessing a site that uses a let's Encrypt certificate...
https://talk.peercoin.net/
When I'm connected to wifi the site loads fine on my iPhone, however when it says LTE the site won't load at all. I just get the Safari cannot open the page error.
However, if I'm using wifi and I add the www. to the URL like this...
https://www.talk.peercoin.net/
I get a error message that says Cannot Verify Server Identity. Here is a screenshot...

If I click on details it says Let's Encrypt is "Not Trusted," see another screenshot here...

I've been trying to figure out if this is an issue with the website or if it is an issue with iOS. Any help would be appreciated, thanks!