What is the exact description of the charge you see on your credit card statement? That may be the easiest way to determine who is actually billing you and then you will know where you need to go to cancel.If it say apple.com/bill or something similar to that then it is Apple billing you and if it has Fitnexa anywhere in the description on your credit card statement, then you know they are billing you correctly.
Since you don't see it in your Subscriptions that are managed by Apple, then that is also a good indication that Fitnexa is billing you directly. They would be billing you directly if you started the subscription on that companies website, and if you check on their website, they also have the steps to take to manage those subscriptions including cancelling them. See the first section here titled Purchased on Fitnexa:
https://www.fitnexa.com/support/subscriptionPlan