Testing an iOS app using Appium without paid Developer account
I don't know the rules of Appium, or if it's relevant to this question, but you don't need a paid developer account to test on a real device.  You can develop apps and deploy them to your own real device, for example, without paying Apple.

You only need a paid account if you want the app in the App Store or deployed in any enterprise program.)

Per Apple:


  If you’re looking to develop apps for Apple platforms, download the
  Xcode developer tools, SDKs, and resources for free on the downloads
  page. Program membership is not required. If you don’t already
  have an Apple ID, you can create one here.
  
  Apple Developer Program.
  If you’re an individual or sole proprietor/single person business interested in creating apps for distribution on the App Store
  for iPhone, iPad, Mac, and Apple Watch, enroll in the Apple Developer
  Program. Membership includes access to beta OS releases, advanced app
  capabilities, and tools to develop, test, and distribute apps and
  Safari extensions. Developers enrolled as individuals will sell apps
  on the App Store using their personal name.