If the subscription is being managed by Apple, it can be cancelled here:
If you want to cancel a subscription from Apple - Apple Support
Using Apple Pay makes no difference when subscribing to a service directly with a company. It simply provides them with the encrypted tokenized code provided by your bank so your actual account number is not shown to the merchant. These subscriptions are managed by the company and bill your payment method directly without any involvement by Apple. As with any charge to your credit card the process to dispute is the same, first you have to make the effort to resolve the charge with the merchant and if unsuccessful you can dispute the charge with the bank that issued the card. It may be possible that the subscription was started with a different user name, so they are unable to find it and you are unable to log in. Review your emails for any subscription confirmations that may help with identifying the email address that was used and don't forget to check the junk folders.