We completely understand the trouble and apologize for any inconvenience caused.
The "Website payment preference" auto return URL functionality will only work for the payments processed via "WPS (Website Payments Standard)" i,e PayPal HTML button integration payments.
Note - After you've set up Auto Return, you can use Payment Data Transfer to display the payment details to your buyer.
If you have integrated Smart button code integration on your website.
I request to please contact your website developer to pass the Redirection URLs parameter in your smart button request call to PayPal. PayPal system will auto redirect the customers accordingly to the specified endpoint URLs.
Specify the return and cancel URLs:
return_url. The URL on your website to which to redirect a customer when he or she approves a payment.
cancel_url. The URL on your website to which to redirect a customer when he or she cancels a payment.
Please refer the below detailed guide links to resolve the issue :