Safari (iPhone, MacBook) gives NSURLErrorDomain:-1017
What are you using to host the website?
The issue is because of:

It looks like your server is offering an upgrade to HTTP/2 even though
the connection is already done with HTTP/2 - which makes no sense. Not
only that, it is explicitly forbidden. From RFC 7540 section 8.1.2.2:

I had a similar issue with a .NET API which was breaking only with Safari/iOS and the solution was pretty similar to HTTP/2 configuration
Please check here:
https://serverfault.com/questions/937253/https-doesnt-work-with-safari
Nginx: Header unset Upgrade; does not work if you proxy your request, use proxy_hide_header      Upgrade; instead.