Hello. Generally, only one DDNS can be added for a home ONT. The configuration procedure is as follows:
Click the tab and then choose from the navigation tree. In the right pane, configure DDNS parameters, including Service Provider, Host Name, Service Port, Domain Name, Username, and Password,

Click Apply
Description
I suggest you consult your ISP and check whether this function has been customized. If it has been customized, ask the ISP engineer to enable it for you because they know which DDNS service is customized,
If there is no customization, then your device cannot be added.
Dynamic domain name service (DDNS) associates a static domain name with the dynamic IP address of its host.
Assume that server A provides HTTP or FTP service and it is connected to the Internet using routers. If server A obtains an IP address through DHCP, or server A is connected to the Internet through PPPoE, PPTP, or L2TP, the IP address is a dynamic IP address. That is, its IP address may change each time when server A initializes its connection to the Internet.
The mapping between the domain name and IP address provided by the domain name service (DNS) server is static, and the mapping does not update when the IP address changes. Therefore, when the IP address of server A changes, users on the Internet cannot access server A with domain names.
With DDNS, which associates a static domain name with the dynamic IP address of its host, users on the Internet can access the server only with domain names.