Replacing D-Link DIR 615 WiFi router with FortiWiFi 60C under Time Warner Cable Modem

I thought it will be easy to just follow the attached sketched diagram and the network will be working. But, no. You have to power cycle the Time Warner Cable Modem to make the FWF60C working. The router will get the DHCP IP from the cable modem on the WAN1 after unplug and plug the power cord of the cable modem.