This blog posts contains some miscellaneous notes about the Flask setup with Nginx and HTTPS. This setup applies specifically to my Centos server.
The Ref.  contains a gist of the nginx configuration file as a minimal working example.
Specifically on Centos, it seems that by default the port 443 is not open. Therefore, even though the reverse proxy is configured properly with Nginx, still one might not be able to connect to the Flask service. If it is indeed a port issue, we can refer to the Ref.  for the method to open port 443 on Centos (refer to the https relevant section there).