UPDATE: The Plex mobile app NO LONGER requires Plex Pass or even a one-time payment to stream. Just make sure the Plex app on your device is up to date and you're good to stream without any extra steps!
For those running older versions of the app:
Plex Inc. only allows Plex Pass users to use their iPhone & iPad app for free. Learn about Plex Pass here. If you prefer not to upgrade to Plex Pass, there is an easy and cheap workaround.
Open the Plex app, hit the hamburger menu ☰ then the gear icon, then “Upgrade”, and from there you should be able to purchase an upgrade for the app through the prompt at the bottom for a buck or two. It’ll process it through the App Store and should only take a second to do. Once that’s done you’ll have full access to the Plex app and its feature without needing Plex Pass!