Adding modem

Not much to go on there bud....Got more details / model numbers / what exactly it is that you need to do (are trying to do) ??

I'm trying to connect a router to my modem. Xfinity gave me a yellow ethernet  or internet cable that looks like a phone line end. I need a longer cable than the one xfinity tech left but I need to know what type of cord that is.  I purchased a cat6 cord but that cord wouldn't work. 

This is the cord I'm look to replace. 

Are you direct connecting to your modem currently or only using wifi?  Is it the wifi modem xfinity provides?   Your internet should come to your modem from the 75 ohm(round connector).   So to run a wifi router off that, just plug the cat 6 cable to the internet plugin on the back of your main modem, then hook that to the input(the connection that doesn't have a number next to it) on the router.  On the back of the xfinity modem is a port for a phone line, but that's if you are using it as a home phone.  You want to hook up to the wider one that doesn't have a number next it it.  It should say internet.   Sorry, making a short story long.     75 ohm connector to the back of your xfinity modem, cat 6 cable from the internet port on the modem to the internet port on the wifi router.   You will need to follow the instructions on the wifi router to get the wifi to work.  It should involve you typing a number like 192.168.0.1 where you normally put in the website address then follow the setup instructions to make it secure.