To enable the Plex Media Server repository on Ubuntu only a few terminal commands are required. From a terminal window run the following two commands:

First we need to add some NVIDA packages for doing encoding / transcoding etc.

ubuntu-drivers devices  (will tell you what version to use)
apt install nvidia-headless-5xx nvidia-utils-5xx libnvidia-encode-5xx ocl-icd-libopencl1

You maybe asked to supply a secure password for secure boot driver enrolment Make something up, restart the machine and you will have to enter this password through a direct console - one time only. You can check the driver is loaded with

nvidia-smi

Plex DEB can be downloaded from the plex website plex.tv, and then installed with.

dpkg -i plexmediaserver_1.40.1.8227-c0dd5a73e_amd64.deb

http://10.3.200.1:32400/web

You can also get apt to update plex automagically

nano /etc/apt/apt.conf.d/50unattended-upgrades

Add the repository to the bottom of this section

Unattended-Upgrade::Allowed-Origins {
        "${distro_id}:${distro_codename}";
        "${distro_id}:${distro_codename}-security";
        // Extended Security Maintenance; doesn't necessarily exist for
        // every release and this system may not have it installed, but if
        // available, the policy for updates is such that unattended-upgrades
        // should also install from here by default.
        "${distro_id}ESMApps:${distro_codename}-apps-security";
        "${distro_id}ESM:${distro_codename}-infra-security";
//      "${distro_id}:${distro_codename}-updates";
//      "${distro_id}:${distro_codename}-proposed";
//      "${distro_id}:${distro_codename}-backports";
        "Artifactory:public";
};