Add plex server ppa to ubuntu:
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echo deb https://downloads.plex.tv/repo/deb public main | sudo tee /etc/apt/sources.list.d/plexmediaserver.list
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 97203C7B3ADCA79D
sudo apt update

*Key can change over time, run "sudo apt update" to list new new PUBKEY given when it cant connect to mirror.

Source:
https://support.plex.tv/articles/235974187-enable-repository-updating-for-supported-linux-server-distributions/
Date: March 6th 2023