If it is an in-app purchase managed by Apple, then use this contact number for assistance if you no longer have an Apple device. Those will appear as apple.com/bill on your payment method.
Contact Apple for support and service - Apple Support
It is possible that you also have a subscription directly with the developer of the app, in that case you will need to contact that developer to cancel the subscription.
Just because the company has an app, does not mean that a subscription is managed by Apple. A few examples are Netflix, YouTube, and Walmart that all have apps, but also allow you to have a direct subscription with them that is not managed by Apple.