I seem to be having better luck after the latest Roku update, and having installed a 1050Ti in my dedicated server and enabling NVEnc.

Roku devices force transcoding A LOT under all sorts of circumstances. Transcoding needs pretty robust hardware especially for Blu-ray content. I moved to Nvidia SHIELD devices in my main viewing are and my room, tweaked some settings in Mezzmo for the Roku profile (i.e. disable subtitles entirely) and the Rokus work fine where they are.

I got nuthin' on the inability to find your Mezzmo server. Usually UPnP is only an issue from the outside world. Disabling it can mess with older gaming consoles, or if you don't have IPv6 enabled in your home.