Only Some Devices Have Internet Connection

Ever since purchasing my Xfinity Gateway, only some devices in our house connect to the internet. They all seem to connect to the Gateway but only 5-6 actually have an internet connection. ie, my personal laptop has always had an outstanding internet connection, as have our smart TV's and Apple TV's. But, none of our Alexa devices are ever able to connect, my husband's work laptop is never able to use the internet, nor his iPhone, our iRobot vacuum isn't able to connect, my iPhone connects but then slows down within minutes to the point where I just turn off wifi altogether, etc. 

The final straw is I started a new job today, with a new company laptop, and it connects to the Wifi but has no internet connection. 

Anyone have a solution or seen this problem? Am I really paying this much to only have five devices actually work?!