I have almost the same situation. I have Hass.io with FreeDNS and SSL set up with the Let’s Encrypt add-on which has been working fine. Now I have registered my own domain and would like to add this as well.
I added the new domain to the Let’s Encrypt config as shown below (opened port 80 for the http challenge) and started the Let’s Encrypt service to renew / create the certificates, but I am not sure it worked. When using the new domain name to connect to HA it comes up as unsecured.
I am re-posting this here… as it is more appropriate
I can now access PLEX outside my network and within my network goes to my local HA IP at port 32400.
The one problem I am faced with is when I access outside my network through my Duck DNS domain then it isn’t secure. My HA instance is still secure how can I leverage Lets Encrypt for my plex domain and also use the domain in Duck DNS. I have looked around but couldn’t find a solution