Like Brian, the Add-On wasn’t an ideal (though very slick, IMO) option for my environment due to network isolation, performance and settings that are needed for my Rio / Dell Receivers to function as SliMP3 emulators.
What I did do to address the SSL / iFrame issues was this…
I have Nginx Proxy Manager Add-on providing SSL for my HA instance internally and externally using a DuckDNS domain. I configured a subdomain as a different SSL proxy host (with authentication) for my LMS server. That second proxy addresses the SSL / iFrame issue in the HA interface (when using SSL for HA access). You don’t need to register two DuckDNS subdomains - the Nginx Proxy Manager can initiate the LetsEncrypt certs for any subdomains of your “main”.
To resolve the external DuckDNS [public / SSL] names internally, I run split DNS using the Dnsmasq Add-on as my primary DNS server. Dnsmasq forwards my internal domain to another DNS server I have running. Probably not as efficient as some other configs would be but I had most of those services already configured.