Welcome to the forum. Another poster has mentioned the same issue. Have you tried watching on a new device just to see if it will let the device be added onto your list anyway? If it doesn't and you've not been able to add any new device this month, then have a word with Live Chat, explain what's happened and see if they can sort something out for you.
To reach live chat go to the following help page
NOW are looking for users to join their Customer Feedback Group and help improve the service. If you're interested read more here and sign up.