Its not clear how the coverage map is generated. Its likely just a mathematical process that says if a tower is within a certain distance then you should expect 'great'. Obviously OPtus can't confirm actual reception at all locations. You could try a $2 prepaid SIM from another company to test reception. Also if you could try your sim in another phone at the location that might give some info. But otherwise you should contact OPtus and see if they can send out a field technician to check signal strength etc.