You will either need to get the old account working again or you will have to hope you kept your receipt from Apple when you purchased the iPhone from them.
Apple has two related features, Activation Lock ( http://support.apple.com/HT201365 ) and Find My ( https://www.apple.com/icloud/find-my/ ), which are designed to prevent anyone else from using an iPhone, iPad, or iPod touch if it's ever lost or stolen. Deactivation ( https://support.apple.com/HT201441 ) of either requires knowing the Apple ID and password that set the feature, or by placing a special request to Apple to reset the device. This Apple ID information essentially defines the current ownership of the device. If you do not know the required information, or do not have an Apple accepted proof of purchase, the device is otherwise useless to you.
Here are additional resources to recover your account information or if have an Apple accepted proof or purchase:
● Help with a lost Apple ID or password:
• For help with a forgotten Apple ID (this is likely an active email address or one used in the past), click here --> https://support.apple.com/HT201354
• For help with a forgotten Apple ID password, click here --> https://support.apple.com/HT201487
● Apple might reset a device if you can prove to Apple's satisfaction you are the owner. However, Apple will not do so if Lost Mode is set on a device which may have been done by a scammer if a person signed into their device with scammer’s account.
Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. This is done online* at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not provide many details in the article as to what is acceptable proof. Prior to this official service Apple typically required the original receipt** issued by Apple or an Apple Authorized Reseller. Note that while this will let you get use of the device again, it will not help you with lost access to an account.
* There is a special link for this service. To save time you should use it instead of https://getsupport.apple.com/
**Apple recommends you keep your receipt after you buy an Apple device. --> https://support.apple.com/HT202711
