On ElementaryOS 5.1 (Hera) all I needed to do (as with many other newer Ubuntu-based distros) to play DVD-s was:
Pre-requisites (which are probably already done):
sudo add-apt-repository multiverse
sudo apt update
sudo apt install -y gstreamer1.0-plugins-ugly gstreamer1.0-plugins-bad
Insert the DVD.
Open the Files app.
Click your ...