It’s probably because of this: https://techcrunch.com/2021/09/21/lets-encrypt-root-expiry/
What version of iOS?
There are some workarounds posted in this thread: NET::ERR_CERT_DATE_INVALID Error on All Sites
But I’m not sure if iOS is covered there. But in short, this is a likely a consequence of running a severely outdated operating system. If you’re running a newer, supported version of iOS then maybe it’s a different issue.