Look like NameCheap URL update is using HTTPS that should not work for Peplink Dynamic DNS settings (HTTP).
https://www.namecheap.com/support/knowledgebase/article.aspx/29/11/how-do-i-use-a-browser-to-dynamically-update-the-hosts-ip
We would suggest to use Peplink IC2 Cloud Find My Peplink Service.