URL is not pointing to https on iframe

[Geo P.C.]

In our site we are loading a calnedar api function that works on http (
cdn.instantcal.com) . While loading this site on our wordpress site with
https its not working and getting an error:

"Mixed Content: The page at '
https://www.geo.com/wp-admin/post.php?post=362&action=edit' was loaded over
HTTPS, but requested an insecure resource '
http://cdn.instantcal.com/cvj.html'. This request has been blocked; the
content must be served over HTTPS."

In order to fix the Mixed Iframe Issue in our Nginx Proxy server we
configured a new site on https calendar.geopc.com and that proxies to
cdn.instantcal.com.

server {
  listen 443;
  server_name calendar.geopc.com;
location / {
    proxy_pass      http://cdn.instantcal.com;
                        proxy_set_header Host      cdn.instantcal.com;
                       proxy_set_header X-Real-IP $remote_addr;
        }
        }

Then In Iframe we given the url as

But in Iframe we are getting the same error

Mixed Content: The page at '
https://www.geo.com/wp-admin/post.php?post=362&action=edit' was loaded over
HTTPS, but requested an insecure resource '
http://calendar.geopc.com/cvj.html?idcloseable=0&gnavigable=1&gperiod=da'.
This request has been blocked; the content must be served over HTTPS.

When we directly access the url calendar.geopc.com on https its working
fine on https. But please let me know whats the issue? Is it on Iframe or
on Nginx. Can anyone please help us?
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.nginx.org/pipermail/nginx/attachments/20161003/4385923f/attachment.html>