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, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.
If you have the necessary documentation, Apple have recently introduced a new portal though which you may be able to request release of an Activation Lock:
How to remove Activation Lock - Apple Support & start an Activation Lock support request.
However, this isn’t necessarily the end of the story...
A close Family member or Executor, with the required supporting paperwork, can make application to Apple to be granted access to the Apple accounts of the deceased:
How to request access to a deceased family member's Apple accounts - Apple Support
Quite often, close family also have sufficient knowledge - and access to likely recovery paths - that access to the AppleID account of the deceased family member can be recovered. Having done so, it is then possible to release the Activation Lock that prevents reuse of an iPad/iPhone.
The following guidance is written from the perspective of the owner - but a family member will get the idea as they run through the process.
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If your device is Activation Locked, this help page will provide most of the information that you’ll need:
Activation Lock for iPhone, iPad, and iPod touch - Apple Support
If you have forgotten your AppleID or associated password, these can be recovered here:
https://iforgot.apple.com/
To recover your credentials, you’ll need access to any one of the following:
- Your primary email address mailbox that corresponds with your AppleID
- Any of the secondary/recovery email addresses that should be configured for your AppleID account
- Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with your AppleID
Unless you 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 your AppleID:
If you forgot your Apple ID - Apple Support
More information about recovery of your AppleID password:
If you forgot your Apple ID password - Apple Support
When you recover access to your credentials, you 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/
Having recovered access to the AppleID account (and therefore the AppleID username and associated Password), Activation Locks can be removed. There are several routes to this, but for brevity, I’ll refer you to this support page:
Turn off Activation Lock – Apple Support
If the passcodes of any devices are unknown, the data stored on the device itself is now gone - but from your question I surmise that data recovery is not you immediate aim. That said, if the device(s) have an associated iCloud backup, you can restore the backup data to the device. To do so, you would need to follow this procedure:
If you’ve forgotten the passcode on your iPad, or your iPad is disabled – Apple Support
You’ll need access to either a PC (with iTunes installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of MacOS). At the end of the process, you’ll have the option to restore data from the backup.
If your intent is to wipe/reset the device(s) for reuse by a new user, you would be best advised to prepare them properly:
What to do before you sell, give away or trade in your iPhone, iPad or iPod touch – Apple Support
I hope to have covered all the likely scenarios - and similarly hope that this guidance is helpful in fully resolving your current difficulties.
