How to block apps from tracking your device?

Hi,

So I dowloaded this app and signed up on it. Then deleted it, erased all data on my iPad, re-downloaded it again and tried to sign up and it just said "sorry, you've already signed up on this device." how exactly does it know my prior history with the app when I deleted all data and files and even downloaded it with a different Apple account while hiding my server with a VPN?

I really want to sign up on the app again, but it says only one "sign up" per device. How do I get around this without having to switch to another phone/iPad?
Added (1). I'm not using the same username or email when I sign up.

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