Can you check under My Account/Bills and Payments, and see what it says there? You should see 3 month’s entries at zero, from your Pass, and then this charge in the latest month.
Do you have your email from NowTV at the time of cancellation, confirming that this was successful?
Web browsers are known to block NowTV Chat for various reasons; ad blocking software, popups blocked, cookies disabled and so on.
The answer is to use as plain vanilla a browser as you can for this purpose; not your possibly carefully configured default browser, which you should not disturb, but one of the others that you haven’t tweaked.
Edge perhaps, or Chrome, or Firefox, or Internet Explorer, whichever one you don’t normally use out of these, so it hasn’t been configured away from default and is more likely to let you reach Live Chat.
Then, armed with the information gathered above, you can discuss with them why NowTV have charged you, and seek a refund.
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.