Did you initially sign up for MysteriumVPN service through the Apple App Store? If so, your subscription is managed through Apple, and they actually handle all the billing process. Here's how to contact Apple directly to cancel your subscription.
Click on Billing & Subscriptions.
Click View, Change, or Cancel Subscriptions.
Click Continue.
Scroll down and choose to contact them via chat/message (recommended!) or email.
Provide Apple with the details of your subscription ↗️ to cancel it.