Comment 5 for bug 1872118

MM , Thanks I install the isc-dhcp-server from the Bionic-Security repository, and it appears to work.
For anyone else here is how to install an older version of the server,and then block updates to the focal version

PLEASE USE AT YOUR OWN RISK

#Backup your old config
sudo mkdir /etc/dhcp.20.04
sudo cp -r /etc/dhcp /etc/dhcp.20.04

#Remove your old isc-dhcp server
sudo apt remove isc-dhcp-server

#Add the old Bionic repository
add-apt-repository 'deb http://security.ubuntu.com/ubuntu bionic-security main'
sudo apt update

#Install isc-dhcp-server from the bionic-security repository
sudo apt-get install -t bionic-security/main isc-dhcp-server
sudo systemctl restart isc-dhcp-server

#Block upgrades of the package from the Focal Repository
edit /etc/apt/preferences.d/repo and add the following lines

Package: isc-dhcp-server
Pin: release a=bionic-security
Pin-Priority: 900