How to cancel a monthly charge that is not showing on my subscribition

If its not listed in your subscriptions then its not tied to your Apple ID.

Where are you seeing this charge? Is it showing up as "apple.com/bill" on your payment method's statement? Or as something else?

If it appears as "apple.com/bill" and you are the family organizer in a Family Sharing group or you share your payment method with any one, look for the subscription on their account.

If it shows up as something other that "apple.com/bill", then it is not tied to your Apple ID and would need to be cancelled that same way it was started or through the merchant's or provider's website or platform.

The following links should help you to both identify and manage any recurring Apple service subscriptions associated with your AppleID account:

If you want to cancel a subscription from Apple - Apple Support

Manage App Store purchases, subscriptions, settings, and restrictions on iPad - Apple Support

Subscriptions and Billing - Official Apple Support

View your purchase history for the App Store, iTunes Store, and other Apple media services - Apple Support

Subscriptions purchased outwith the Apple App Store must be managed/cancelled directly with the service provider.