> The page itself is served in both HTTP and HTTPS, can be visited in and outside the LAN.
So why can't you use one of these options:
1. Redirect all users to HTTPS, since you apparently already have a globally valid certificate.
2. Rewrite the tags to use HTTPS only when the page is loaded over HTTPS.
3. Set upgrade-insecure-requests only when the page is loaded over HTTPS.