It is fit for purpose, they just cannot guarantee 100% coverage 100% of the time; there will be dead spots, maintenance work, and faults, just like every network in the world. The SureSignal is a bonus where if you are in a no/bad signal area you can choose to use the service.
In short, they are not breaking the contract because they are supplying you with a workable network, just you happen to be in the wrong place. Furthermore, if you didn't change your mind within the returns period, then you've accepted it as is, and cannot change the goalposts midway into the agreement.
