Is there a Coax to Etthernet (RJ45) adapter?

So a friend moved and gave me his cable modem/wifi/router (which he owned). Since I already have one, I cannot use a second one. But I have another friend who is in need of a wifi/router.

The Internet port accepts coax. (The local "home" ports are RJ45). 

Is there an adapter I can get that would allow me to plug into the coax port of the modem and have an RJ45 output? 

The goal would be to have the "uplink" coax port accept RJ45 Ethernet connection.

Thanks