tv Streaming keeps dropping

I’ve got the fastest internet speed Xfinity offers but when we’re watching a smart tv app like Hulu, Netflix, or any other, the screen keeps freezing saying bring have WiFi connectivity.  I’m also having this issue with wireless printers/laptops as well as our security system.  I’ve replaced our tv and still haven’t owning. Xfinity says our connection is great and must be an outside issue but it ve had this problem for over a year…even got boosters installed on each floor and it’s worse than better.  Suggestions??? I live in a development that has underground cables that are over 30 years old.  It is in a valley so reception has always been an issue with cellphones etc.  I need help