And older phones might have certificate stores that can't be upgraded and have already expired.
I believe you can work around this using another machine as an SSL proxy - though setting that up is beyond my ability. Perhaps someone else can elaborate?
https://dev.to/suntong/squid-proxy-and-ssl-interception-1oa4
- looks like it might be a useful guide for setting it up as an SSL proxy.
I took a deep dive into this after I was unable to access my blog on my iOS 6 device. I concluded that I don't really need a ssllab's A. It is much more likely someone will try visiting my blog with a older device than someone will MITM one of the visitors.