Step 1. DNS
First you need to visit your host provider and go to Manage your Domain Name System (DNS) settings. For example I use Cloudflare to manage DNS. There you will need to add a new CNAME record. So for example I want a subdomain http://helloworld.yourdomainname.com.
To do this you add new DNS Records:
TYPE – CNAME
NAME – helloworld
VALUE – @
TTL – Automatic
Status – DNS Only
Step 2. Router
Make sure your router is setup with port forwarding point to your Synology server.
Step 3. Synology server
Finally login to you Synology box and go to Control Panel > Application Portal > Reverse Proxy. Here you will need to add new Revers Proxy Rules. The rest is straight forward. So now you can check is your new domain URL works. If not, go back and check all three setting are correct.