Ciao 

What is the FQDN used in Horizon Client to connect (https://myserver.com:443 or https://sina.com:3088)?

Did you try to disable the Horizon client SSL check and re-try the connection?

Is the certificate installed in the repository of the local computer of the connection server to which the VDM friendly name has associated the one used externally to connect?

From the local network, you are able to connect?