Are you sure a hacker is doing this? But if they are, log into the router, as in the Guide that came with it, and change the password from the default, so they can’t log in again.
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.