DDNS (Dynamic DNS) is a service that maps Internet domain names to IP addresses. DDNS serves a similar purpose to the Internet’s Domain Name System (DNS): DDNS allows anyone hosting a Web or FTP server to advertise a public name to prospective users.
Unlike DNS that only works with static IP addresses, DDNS is designed to also support dynamic IP addresses, such as those assigned by a DHCP server. That makes DDNS a good fit for home networks, which normally receive dynamic public IP addresses from their Internet provider that occasionally change.
If you are running Firewalla in DHCP mode, and want to access your home device using the public IP address provided by DDNS, you have to do port forwarding in order to make it work. Here is the detail instruction: https://help.firewalla.com/hc/en-us/articles/360017789393-How-to-setup-port-forwarding-for-devices-in-DHCP-Mode-