iphone not connecting to wi-fi

My family switched our verizon iphones to Xfinity mobile.  Before switching, my wi-fi on our phones was good but know we are noticing a lag with our wi-fi on our phones after the move to Xfinity mobiel.  Laptop works fine on wi-fi, but not iphone.  I have to turn off wi-fi for me to surf the web or social media sites.  Please advise.