MacOS Server DHCP lease duration change
First enter - to get an overview and the appropriate array_idUUID:

sudo serveradmin settings dhcp


Then (with the proper array_idUUID)

sudo serveradmin settings dhcp:subnets:_array_id:array_idUUID:lease_max = 28800


Example:

sudo serveradmin settings dhcp:subnets:_array_id:A387646D-BBC1-88CD-993E-2D36E15C5089:lease_max = 28800


Then restart the DHCP service:

sudo serveradmin stop dhcp
sudo serveradmin start dhcp


Check the new settings with sudo serveradmin settings dhcp