cannot access website using domain name but works with public ip

I recently set up a blog server (ghost blog) and tied up with my custom domain name. I checked the DNS setting ( A, CNAME et al) and it's been set up correctly.

I could access the website using my public ip and the port number but not using my custom domain name

for example

domain name : www.myblog.com
public up : 217.x.x.x
host access: 217.x.x.x:3500

When i use the external ip together with the port I could access my website. Others could access the website using the ip & port .

I'm not sure where I am going wrong.