edit: i can't believe i have to write this, but - i am not your technical support representative, and i'm not going to reply to your support requests 😖
so this is a bit wild
i've got an oldie iPad 2 running iOS 9.3.5. this has some expired root certificates which results in several apps not working properly. Plex will load, but it won't be able to negotiate with your plex server, for instance.
since apple doesn't offer new iOS updates for it (with refreshed certificates).. it seemed like i was kinda stuck.
as it turns out, and is documented nowhere aside from a post on reddit, you can manually download your own root certificate - and iOS lets you install it without complaint.
if you're on an ancient iOS device, just point its Safari browser to this url: https://letsencrypt.org/certs/isrgrootx1.der
tap Install when prompted - and you've got working certificates again!
this just breathed new life into a 10+ year old iPad, which will become a bedtime plex viewer.