The SB8200 is a modem only device. Most ISP’s only support 1 public IP address. This address is assigned to the modem that then sends that to the router which uses it for private ip addresses on your network (simplified version). for that reason, most ISP’s only support 1 connected device. 

The 2nd can be used for wan port aggregation (speeds greater than 1 gig) or if your isp is one of those lucky one that supports more than 1 public ip address. (rare)