I can't read the full volumes even though I'm a member
I can only read part 1 and it says that part no longer exists because it's already being sold.
@Sakuraba Unless a series is on "catch-up," the subscription only gives you access to the books that are currently being translated (and part 1 of each existing pre-pub for whatever reason - as a preview, I guess). The pre-pubs for a book expire not all that long after the epub is released. So, they allow you to keep up with a series but don't give you the ability to start a series or catch-up unless it's brand new.
Now, JNC puts a few series on "catch-up" every month, and those have all of their existing pre-pubs available for that month (after which, they're unavailable again), so if you wait, you can eventually start a series that way (though depending on the series, it could be next month or over a year from now - or probably never if the series is complete). Otherwise, you'll need to buy the books to catch up with the current pre-pubs, and then you can read the pre-pubs from there.
The whole point of the pre-pub stuff is to give you access to the books as they're being translated (but not fully edited), not to give you access to every book that JNC has ever translated. They'd probably have to charge a lot more if it gave you full access, since they do ultimately need people to buy the books if they want to be profitable.