This was said by a staff (Rahul Balaggan) in 2019 regarding premium credits without after membership expires:
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.
The last is easier than e-mailing, of course.