Due to CloudFlare only support some ports, I need to change Synology NAS admin console to use port 443 instead of default 5001. However I cannot get it done using Synology web admin, because port 80/443 are reserved for PHP website hosting, even I turned them off...
Google and found out that I can hack the config file directly.
Login using SSH, modify
/etc/synoinfo.conf file, change port from 5000/5001 to 80/443.