TV Cable Box Setup

I’m receiving the RDK-03013 error code. 
The setup manual says I have to connect a coax cord to the wall port and cable box port, then connect another coax cord from the cable box port to my tv port. The equipment pack only came with one coax port. 
Furthermore, I only have one coax port in my apartment which is currently already being used for the Xfinity Router. 
what’s the alternative or what am I doing wrong. 

note I already tried using an Ethernet cable to connect the cable box to the router.