dear @Ron, thank you so much. I have set sl.cosd.com as the primary domain, and now it is serving that subfolder site's content under that primary domain.
The other domain, st.cosd.com, still redirects to 54.76.48.242/st without our mapping having any effect.
What else could I have missed? Do I need to reinstall the site using subdomains instead of subfolders, as suggested in that other thread?