Coverage tracker completely inaccurate

I got my phone just over a week ago and checked the coverage in my postcode, both in store and at home.  They both rate 4G as “Very Good”.  However, when I phoned you as I could not make or receive a call, even using WIFI calling, your colleague informed me that there is no 4G in my area and I would have to drop to 3G.  How come they know this but your coverage checker does not?  Even with 3G and WIFI calling, I am struggling to make an recieve calls so will have to return the phone and sim within your 14 day moneyback arrangement.  Very disappointed and feel that I have been totally mislead by you at point of sale.  I did buy at Carphone Warehouse but they were using your checker and information so it is not their fault, it is clearly ID’s.