you most likely need to pass a cookies file to authenticate How do i pass cookies to yt-dlp

If the video is actually encrypted (like a streaming service) there are a lot more steps to do to strip Widevine

Wonder how big the mangadex catalog is, 15TB?

Blutopia, a private tracker. Would take quite a while to get there so i would suggest finding a good samaritan to download and share the files with you