I presume that you don’t have a fixed IP, do you ? So, first, you have to configure the DDNS feature of your Synology to let people have access to your NAS using a friendly hostname like http://Paul.Diskstation.me/wordpress. DDNS can be configuration via the “Control Panel” of the Synology. It’s quite straightforward and simple.

I presume that you have a modem/router (both integrated in one device) ? So, next, you have to forward the port 80 of your modem/router on the port 80 of your NAS. This kind of configuration must be made via the administration interface of your modem. This service is often named “Port Forwarding”, “NAT”, “Server” or “LAN”… but depends on the brand you have. Look for “Port Forwarding” and “NAT” on Google to get some insights.

If you are lucky, i.e., if your modem/router is compatible, you can even let your Synology do the “port forwarding” configuration for you, via the “EZ-Internet” feature available next to the “Control Panel” of your NAS.

A big picture (more complete) is available here: https://www.synology.com/en-uk/support/tutorials/456.