The SB8200 is a cable modem and will not DHCP private addresses with it is online.  You normally would connect Wi-Fi router to the SB8200 and the Wi-Fi router will receive public IP from the ISP.  

When SB8200 is not connected to ISP, it will DHCP a private address (192.168.100.x) to a connected device to access the Web management portal at 192.168.100.1.

Then how to place your router behind the modem? Because after provisioning the modem with my ISP, the router is not receiving a DHCP address.

After the modem is provisioned, make sure the LEDs are solid (modem is connected to ISP).   Connect only the router to the SB8200, can only one Ethernet port on the SB8200 since one WAN IP is provided by ISP. 

From there, you can either release/renew wan IP address on the router.  Or reboot the modem first, wait until all the LEDs are solid, and then reboot the router.   

​@Netopia is exactly correct. While the SB8200 has 2x ethernet ports, most ISP’s only support 1 connected device to it. So if you used a pc/laptop to setup the SB8200, disconnect everything from the modem and power it off for around 10 minutes (most ISP’s only need a minute or 2 but some longer). Then connect the router to port 1, power on the modem and give it a couple minutes, then power on the router and go through setup.