@FayeABrown you need to include your mangadex_session in the header as a cookie. You can get your mangadex_session from logged-in cookies.
curl -H 'Content-Type: application/json' -H 'User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko)...