This is the situation , even though you have 2 Ethernet ports on the back of your SB8200 modem, you will be able to connect one device at the time and the reason why its because the modem is getting just 1 valid IP address If you want to use both ports at the same time you will need to contact your ISP and request a secondary IP Address

LIke @SURFboard Moderator moderator says, its related to your isp. The vast majority of ISP’s only assign 1 public ip address. Its then on the router to take that public ip address and assign private ip addresses to devices on the network. Unless you pay for a business class line or your ISP lets you pay for extra public ip addresses (most don’t on consumer connections), then you’ll be limited to a single device connected to the modem.