When using Humly connect function, it is a must to accept the self-signed certificate to be able to connect to the device over VNC service.
The connection is established over an IP address “The Device IP Address” not using FQDN. Since the browser is connected to two destinations, the first is the connection to HCP, the second one is the connection to the device. Then it must show an insecure connection warning.
But this does not mean that the connection between the browser and the Humly Control panel is un-secure, this connection is always secured by Humly Cloud TLS if you are using Cloud Services, or it should be secured by your installed certificate if you are using the on-prem solution.