Ethernet not working

I have a centurylink router and modem that was built in to the apartment when I moved in. I was using that for a while and was having issues due to only getting wifi speeds of up to 15 Mpbs, while the ethernet was at 400+ Due to needing faster wifi speed and not being able to troubleshoot, i decided to use xfinity, rented a router/modem and now am using that. the wifi connection is much better, but the ethernet connection wont work at all. I need to figure out how to get the ethernet working on xfinity. I tried going back to centurylink and restarting my service and the ethernet worked immediately and I didnt need to touch anything. I dont want to pay for century link though id prefer to stay with xfinity. How can i get my ethernet cable to work with xfinity?