The serial number won’t be of any help to you.
Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be turned off:
Activation Lock for iPhone, iPad, and iPod touch - Apple Support
Turn off Activation Lock – Apple Support
As you will discover, unless you are the owner of the AppleID that was last used to activate the device, or unless you can secure the cooperation of the previous owner, you’ll not be able to defeat or bypass the Activation Lock. Without documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple won’t assist - and nobody here will be able to help you.
The owner of the AppleID that previously activated the device - who has forgotten their AppleID or associated password, will be able to recover their credentials here:
https://iforgot.apple.com
To recover your credentials, you’ll need access to one of the following:
- The primary email address mailbox that corresponds with the AppleID
- Any of the secondary/recovery email addresses that should be configured for the AppleID account
- Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with the AppleID
Unless the AppleID account has not been fully/correctly configured, or has been seriously neglected, recovery of the AppleID and password should not be difficult.
More information about recovery of the AppleID:
If you forgot your Apple ID - Apple Support
More information about recovery of the AppleID password:
If you forgot your Apple ID password - Apple Support
When the owner recovers access to their credentials, they would be well advised to log-in to your AppleID account from a web browser - and verify/update any email addresses and trusted telephone numbers:
https://appleid.apple.com
As an alternative, if the original sales invoice that clearly shows the serial number of the iPad is available, their is rumour that Apple might accept this as proof-of-owvership - and in such cases might assist with reset of the device.
I hope this insight and guidance proves to be helpful in resolving your problem.