I recently had to stop my membership due to tight finances, and when I went to use one of my many (15+) premium credits, I could not find out how. Going to the usual "Buy EBook - Premium" brought me to a re-sub page.
Can premium credits only be used when subscribed... ?
Short answer: it is easier to use premium credits when you are subscribed.
Long answer: it is still possible to use premium credits, you can email support@J-Novel.club and have them manually take the credit and add the desired book to your account.
Or you can manually go to the books redemption page and by typing out the URL and redeeming that way.
@rahul-balaggan Thank you for answering. Hopefully they address this issue.