Is it possible that you have your broadband on a different account from your streaming?
Go to My Account above/Manage Account/Account Summary (at the top, with the hamburger/licorice allsort) from where you will be able to access your Broadband details; this will either be a success, or it will look like you haven’t got a broadband account, which means you do have a separate account for this.
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.