The Synology DSM has a handy built-in updater for dynamic DNS (DDNS). It makes a lot of sense, given the Synology is always on and connected to the internet, keeping a custom domain pointed to the right IP at all times.
This is a companion discussion topic for the original entry at https://brettterpstra.com/2021/08/26/custom-urls-for-your-synology-with-namecheap