I have a 2820VN router with one ADSL and one cable connection. Both connections are set to "Always On". The problem is that if my cable internet connection stops working due to a problem at the service provider, the router still sees the cable modem as "connected" albeit there is no internet connectivity and I cannot get online from any PC. I have to physically unplug the ethernet cable from the modem to the router and then the ADSL kicks in.

How can I get the router to automatically switch to alternative connection?

My firmaware is 3.3.3_232201.

Any guidance would be appreciated.