How to have backup internet connection

As ptcl is very unreliable for me I am thinking of a backup internet connection like wateen/wi tribe but I am unsure how my internet connection will be switched in case ptcl fails? I don't want to switch to backup connection manually, I have heard for multi wan port routers.. do you have info about good solution?

Well i have been using Wateen from almost an year and can guarantee this that have faced only a little or so down time up till now in worst weather conditions.