Recent firmware on the USG (tested on 4.5.3dev, but should work on recent 4.4x also) allows for native configuration of CloudFlare DDNS without a third party service (like the ever popular DNS-O-Matic).

The only catch [for now], is you have to use config.gateway.json (this file is on/needs to be created on the controller in the applicable site) to accomplish this (as of Controller 5.9.4).

If you need further information please see UniFi Advanced USG Configuration for help about where to create the config.gateway.json.

PLEASE NOTE:

• Prior to creating the .json file on your gateway, create the file in a text editor, such as Atom or Notepad++, and then validate your file at a JSON validator site such as https://jsonlint.com/.
• If the syntax fails verification, and you provision it on your gateway, it could cause reboot issues.
• If your unsure, reach out to the Ubiquiti Discord Chat or the UBNT Community Forums prior to saving/provisioning for support.

Below is an example of the config.gateway.json needed to make this work:

{
	"service": {
		"dns": {
			"dynamic": {
				"interface": {
					"<WAN interface eg eth0>": {
						"service": {
							"cloudflare": {
								"host-name": [
									"<insert A record name here eg. usg.example.com>"
								],
								"login": "<CloudFlare E-Mail>",
								"options": [
									"zone=<DNS Zone eg. example.com>"
								],
								"password": "<CloudFlare Global API Key>",
								"protocol": "cloudflare",
								"server": "www.cloudflare.com"
							}
						}
					}
				}
			}
		}
	}
}

The above did not work for me, on 4.4.36.5146617. But the following does

1. Upgrade ddclient to 3.9.0 (from 3.8.3)
2. Install the required dependency libdata-validate-ip-perl (Requires adding remotes, so make sure do not do upgrade of any packages)
3. Make sure the subdomain you want to automatically is set to any value at cloudflare.
4. Do the same config as above but skip server (as in remove that part of the json)
5. Now it should work.

If it complaints about

 Invalid Value for keyword 'ip' = ''"

don't worry it has nothing to do with getting the dynamic dns working.