If you subscribe to Eso plus it will give you immediate access to the previous Chapters as they have now converted into DLCs. Do keep in mind you will only have access while you have a current Eso Plus subscription. Furthermore, on occasion these previous Chapters (now DLCs) go on sale during events or Crown Store showcases, so something to look for. These DLCs are available to purchase in game through the Crown store for Crowns if you wish to purchase them individually.

If you wish to buy the Blackwood Collection it would give you access to the missing Chapters that you do not currently have access to. By purchasing them outright in this manner it would give you permanent access to them, regardless if you subscribe to Eso Plus or not. Additionally by choosing the Blackwood Collection - Collectors edition, it would give you access to the previous Chapters Collectors edition items, if you did not previously go this route and just got the regular editions of your previous Chapters.