Xbox 360 help best answer to whoever helps me?

two weeks ago a comcast guy swichted my modem to onother one it is a touch stone telephony modem and i cant go to xbox live every time i go it doent p the dns test it pes the ip but not the dns i tried puting in manual i typrd every thing and it only confirms the ip adress if i put it on automatic nothing pes how do i fix this i have an xbox 360