Cantiara Purchasing privilege only means that you are paying for the opportunity to unlock and read X advance chapters ahead of the regular readers.
If you buy the 10 advance chapters tier, it doesn't mean that you will be getting 10 extra chapters each time you purchase that privilege tier.
You will only get the initial additional 10 chapters when you purchase the privilege tier for that particular novel for the first time. This is to get you to be 10 chapters ahead of the regular readers.
What happens afterward is that you will be getting the normal daily updates, as per usual. If the novel's update rate is 2 chapters daily, then you'll also get 2 chapters of that novel every day, with the only difference that you will still be 10 chapters ahead of the regular readers.
So, for example, if the regular readers are in chapter 10, as a privileged user that has bought the 10 advance chapters tier, you would be in chapter 20. When the regular readers get 2 chapters in the daily update, which means they get chapters 11 and 12, as a privileged reader, you will be getting chapters 21 and 22, and so on until your privilege access expires at the end of the month.
After your privilege access expires, it simply means that you will go back to the chapter where the regular readers are at. So in the case of the example above, you'll go back to chapter 12.
You can still read the privilege chapters that you bought previously. But, if you don't repurchase the same privilege tier (or a higher privilege tier) in the new month, then it would take you some time before you would get the usual daily updates again because you will have to wait until the daily updates for the regular readers catches up to where you were when you were still a privileged reader (in the case of the example above, chapter 22).
So,
If you bought the 10 advance chapters tier in the previous month and repurchased the same tier in the new month, It doesn't mean that you will be getting 10 extra chapters in the new month. You're now exactly where you were in the previous month, and you'll be getting the new chapters update every day, and are still 10 chapters ahead of normal readers.
If you bought the 10 advance chapters tier in the previous month and decided to repurchase a lower privilege tier in the new month, say, the 5 advance chapters tier, you'll have to wait until the daily updates for the regular readers to catch up to the chapter where you left off when you were still a privileged reader for the 10 advance chapters tier. If the novel updates 1 chapter daily, then you'll have to wait 5 days before you would get a new chapter again.
If you were a privileged reader before and decided you don't want to repurchase the service anymore, then you'll have to wait until the daily updates for the regular readers to catch up to the chapter where you left off when you were still a privileged reader.
The privilege service expires at the end of every calendar month, using their server time, which is set to UTC+8 timezone. Calendar months mean January, February, March, April, etc. So the expiry date is not counted from where you bought the service, but literally at the end of every calendar month (October 31st, November 30th, December 31st, and so on.) UTC+8 means that if it's already December 1st in your timezone, it doesn't mean it's already December 1st in UTC+8 timezone. Google the time difference between your timezone and their servers' timezone (UTC+8) and take that gap into account before you purchase the service.