That link tells you everything except that when you haven’t got a global option for subtitles, you have to be playing the programme, and press Pause, in order to get the speech bubble to show up.
It’s hinted at in one of the images in the link, but never made explicit 😢
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.