The only way to reset the passcode is with a computer. Or with the assistance of Apple or an Apple service provider, or a trusted person with a Mac or Windows system that can perform the reset on your behalf.
And if they didn’t clear the passcode, they probably also didn’t clear the Activation Lock, and if they didn’t clear Activation Lock you will want to return the device for a refund, or recycle it. Activation Lock is a theft deterrent k and Apple will not assist with bypassing it, absent legal paperwork or proof of purchase with serial number and matching photo ID.
The following two links have more info on clearing the passcode, on Activation Lock, and on purchasing devices which turn out to have been lost or stolen:
How to Unlock an iPad or iPhone? - Apple Community
The All Too Common SAD Reality of Buying a Used iPad/iPhone