The only way to cancel a subscription using the method provided by Apple is when the subscription was made using an in-app purchase so that Apple processed the payment. Some companies want you to subscribe on their website so that they can avoid paying Apple a commission to process the payment and also so you cannot cancel the subscription through Apple. Whenever you subscribe outside of an app, you are dealing with that company directly, bypassing Apple's ability to manage the subscription. It makes no difference if that company also has an app, the only difference is where the subscription was purchased.
In the future, only use the in-app purchase for subscriptions if you want to be able to easily cancel that subscription. What you can do now is write a review for the App on the App Store to alert other users who may be interested in downloading it. Those users will not see your post here, but will be able to see it in the App Store if they choose to read the reviews.
You can also use the Report a Problem link to express your concerns about the app. While this link talks specifically about requesting a refund, one of the options you can choose is to report an app as a Scam or Fraud. Apple has removed apps where enough people have reported a problem with it or it had received too many negative reviews.
Request a refund for apps or content that you bought from Apple - Apple Support
