Nobody answered yet? Tut.
A strong possibility is that your broadband is on a different account from your streaming.
You best bet is to call the broadband number (I can’t give it to you, but others here seem to be able to hand it out like sweets without sanction, so search a few threads and you should find it) and discuss it with them.
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.
If you take an offer that comes with a ‘free trial’ of Boost or another Membership, Cancel them straight away. You will still get the full duration of the trial, and if you decide you do want to continue with one or another of the trial products, just revisit the Cancel, and UnCancel it.