This Repository can be added to your system manually by copying the lines below and adding them to your system's software sources.
sudo wget https://archive.darkgrave.de/dgrepo-archive-keyring.asc
sudo apt-key add dgrepo-archive-keyring.asc
sudo add-apt-repository "deb https://archive.darkgrave.de/unity/ YOUR_UBUNTU_VERSION_HERE main"
sudo apt update