Yesterday my internet connection went down.  When it did my entire internal deco network stopped working.  I lost access to my security cameras, my media server, my NAS, etc.  None of which is dependent on an internet connection.

I emailed tp-link support who responded that this is by design and i should talk to my ISP for a better connection (we get 200 inches of snow here every winter, internet outages are unavoidable where i live).

This is not only inconvenient, but also unsafe as my internal security system components can't communicate with the network down. 

Ive never even heard of a router failing to route internal traffic because  of an external link going down.  Surely the email support team got it wrong.  How do i prevent this from happening again?