C7800 Nighthawk X4S AC3200 used as Router without Coax Cable

Is there a way with my  C7800 Nighthawk X4S AC3200 to configure it to be only a router where I can plug an ethernet cable into one of the internet ports and allow it to connect to another modem and then broadcast the Wi-Fi signal for the internet that comes from an alternate ISP?

 

I want to use this as a router and connect it to a modem from fiber optics that has an Ethernet converter to then plug an Ethernet cable to connect the two devices, but need to know how to configure the Internet ports to be a receiving port and then broadcast the Internet signal wirelessly.

 

Previously, I had Xfinity/Comcast as the ISP, will I also need to change the internal settings to change?

Unfortunately the answer to your question is no.  The C7800 can be configured as a modem only with no router function, but not the other way around.