SSL not working on Windows 7 running Chrome

Might be failing for a different reason. And, the "fix" for Safari would be different than for your client using Win7 and Chrome. But, for iPhone 5S, what version of iOS has that phone been updated to?

Let's Encrypt had its DST Root CA X3 certificate expire on Sept30. It is still part of the default chain (the "long chain") and is what your server uses. This was done to provide best support for older Android devices but other older devices may not work as well. If you need to support a wide variety of older clients you might be better off switching to a different Certificate Authority (CA). Something like ZeroSSL or another free CA.

See here for more explanation