Forget the indignation for a minute, but why should Argos take it back? Is it their fault "the signal is wrong?
As vic says, they are under no obligation to give you a refund, nor are you entitled in law to one, unless you were absolutely specific about it's intended use, (which, in this case would mean you discussed the exact location of the article's use.)
But then, this is Argos - if you want cheap prices then you can't expect knowledgable sales staff with time to discuss your individual needs.
If you are prepareds to pay a bit more, go to a small, independent store who will be prepared to spend the time to guide and advise - and may have local knowledge eg. signal blackspots and be able to advise accordingly.
Your best bet with Argos is to be polite and fish for and exchange or credit note - you have absolutely no right to a refund.