Can't see Secure Connection option

Hi, I have started having stuttering on my Chromecast when streaming from Plex via iOS app recently. After investigation it seems that changing the Secure Connection option on the Settings>Server>Network tab is something to try.
However, I cannot see this option on my Plex Media Server. I am running version 0.9.16.4 and it says it is up to date. The only option I can see when I hide advanced options is Enable local network discovery (GDM).
Even when I show advanced options I cannot see anywhere to select different options for secure connection. I have tried deleting the version of the Media Server and re-downloading it but this doesn't change things.
Not sure what to do next but would be grateful for suggestions.