Once the subdomain has been created, you must link it to a dynamic user.
Note: If you already have a dynamic user created, you can link it to as many domains/subdomains as you want. When you update the IP, it will be updated across all records using that same user.
To assign a user to the subdomain, click on the icon shown in the following image:
If you do not have any dynamic users, you can create one using the available form.
Once created, click Save data and the user will be automatically assigned to the subdomain.
To update the IP from the web interface, click the icon shown in the following screenshot: