While most network appliances you purchase nowadays often come with their own DHCP server, rolling your own gives you ultimate flexibility. Some of these built-in DHCP servers are full-featured and come with everything you need, while others may contain just enough features for it to function, but nothing truly exciting. Ubuntu servers make great DHCP servers, and rolling your own server is actually very easy to do.
First, the server that serves DHCP will need a static IP address. This means you'll need to configure Netplan with a static IP assignment. A static lease won't work here, since the DHCP server can't assign an IP address to itself.
If you have yet to set a static IP, Chapter 4, Connecting to Networks, has a section that will walk you through the process.
Once you assign a static IP address, the next step is to...