How is everything set up. Is it set up for load balance or smart link back up? Have you confirmed that the cable connection can actually get online? Kind of confused with some of your testing.
As long as you have it set to Smart link backup everything should work fine. If your Primary is DSL and the DSL goes down it will fail over to your 2nd wan which is your cable and when the DSL comes back up it will swap back to your primary automaticly without having to do anything.
Hope that helps