I use the free cloudflare DNS service for my private domain. I also want to use this for my dynDNS setup. I the last year ubnt have updated the ddclient on the edgeOS for the cloudflareV4 API. Now the setup work easy out of the box.

Just you the following snippet:

set service dns dynamic interface pppoe0 service custom-cloudflare protocol cloudflare
set service dns dynamic interface pppoe0 service custom-cloudflare host-name FQDN_to_update
set service dns dynamic interface pppoe0 service custom-cloudflare login CLOUDFLARE_USER
set service dns dynamic interface pppoe0 service custom-cloudflare password API_KEY 
set service dns dynamic interface pppoe0 service custom-cloudflare options "zone=YOUR_ZONE use=web ssl=yes"

At the end save the configuration and you are done.