If you did cancel it correctly, then it should not be charging you anymore.
How do you know the 39.99 is for this app specifically?
If it appears as coming from this App directly on your payment method's statement and not from "apple.com/bill" then you need to cancel it directly with Famio, likely on their website.
If it shows up as coming from "apple.com/bill" on your statement, then you can follow the steps in the support document linked below to find the subscription on your Apple Account and cancel it.
click here ➜ If you want to cancel a subscription from Apple - Apple Support
If its not listed or says its expired already, or the charge says its coming from Famio directly, then you will need to go through them to cancel it.
https://account.myfamio.com/contact-form