Setting a static IP address for the Synology NAS would be done through the Synology DSM Web Interface in Control Panel > Network > Network Interface tab. The IP address assigned would need to be in your local network IP range, likely 192.168.x.x with Subnet Mask 255.255.255.0
I assume your router is the DHCP server for your network, so you would want to set that IP address (and any other static IP addresses) as a Reserved address excluded from the pool of automatically assigned IP addresses. It depends on your particular router, but that is often found under LAN Setup > DHCP in your router's web configuration page.
Steve