My guess is you get a block of external static IP addresses from your ISP. You only need to use one of those addresses with a single external NIC on your gateway. You need to setup your external NIC's role as "external" and its connection type as "static", not "DHCP". You also need to enter the netmask and gateway IP address provided by your ISP. You also need to enter the DNS server IP addresses your ISP provides. This is the only way a static IP address from your ISP will work.

If you have the DHCP app installed on your COS gateway, either delete it or disable it, if you're trying to have your router provide a DHCP server for your LAN. I wouldn't think you'd want a DHCP server trying to handle your block of public IP addresses, especially since that's probably just a handful of IP addresses. That's not how a block of public static IP addresses is expected to be handled by an end user. You'd need something like a multi-WAN setup, which the COS Marketplace has available.