How do I solve unstable internet? The Xfinity app won't run a diagnostic if it can't load.

My internet connection has been dropping several times a day for the last few days, for both hardwired ethernet and wifi devices. When the internet is not working, the xfinity app won't load, which means I can't run a diagnostic. The only time the diagnostic can run is when the internet service is restored.  As a result, all of the tests come back with no problems.  

How do I run a diagnostic when the internet is not working? 

I've also bought and installed a Storm Ready, which says it is set up and ready. But when the primary cable goes out, Storm Ready never provides service.