iPhone 11 stuck on Activation Lock after reset

This is a user-to-user forum. You are not speaking to Apple Support.

Under the conditions you describe, there is no way for you to unlock the iPhone, or to get Apple to unlock it. The Activation Lock (anti-theft lock) is doing its job. There is a remote chance that the phone might become unlocked should the previous owner ever take an action that will remotely unlock it – but if I was that owner, and had placed my phone into Lost Mode, the last thing I would ever do is to remove the Activation Lock protecting my phone.

The only help that we can offer you is to

1. Read The All Too Common SAD Reality of Buying a Used iPad/iPhone – Apple Community
2. Return the phone for a refund – if you safely can. (Unfortunately, in many of these cases, once the seller has gotten a buyer's money, they "ghost" the buyer or simply "disappear".)

Apple ID and Activation Lock

iCloud uses Apple ID credentials.

If you do not remember the Apple ID Password then follow the steps given in this Apple Article -> If you forgot your Apple ID password - Apple Support

If you do not remember the Apple ID then follow the steps given in this Apple Article -> If you forgot your Apple ID - Apple Support

If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Learn how to remove a device from a previous owner's account.

If you are not the rightful owner of this iPhone and do not possess the associated Apple ID and password, there is absolutely no way to gain access to the device. Apple’s security framework is designed to prevent unauthorized use, so the only circumstance in which you could unlock the phone is if the original owner willingly provides their credentials. In rare cases, access might become possible if the owner unintentionally removes the device from their Apple ID account, but this is purely accidental and not something you can rely on. In essence, without the Apple ID and password, the iPhone remains permanently tied to its legitimate owner.

It sounds like ‘Find My iPhone’ may not have been disabled or the previous owner’s iCloud account wasn’t signed out. Were you asked to enter a password to do the reset? It’s usually required to do any sort of reset. If you can’t reach the previous owner, you may be out of luck.