Problem setting up cisco router?

i got a cisco linksys router,at its back it has 4 ethernet port and one DSL port for the ISP,d problem is our ISP has a RJ-45 cable and it cant fit on the DSL port of the router,how can i set it up if i cant conect my router from our ISP? is der a thing were i will conect my ISP cable and then conect a DSL cable on it,then to my router DSL port to get a server?