Poor service

I purchased a Samsung fast charging power bank last year october for my upcoming holiday in Europe in December. The power bank refused to work throughout the entire holiday unless surged with electricity from another device e.g. laptop or charger. Upon attempting to return the device (which was sold to me used) the store refused to take it back as it worked in store. Again in South Africa this time, in cold weather, the device refuses to work despite being fully charged. Samsung pavilion has refused to take back/investigate/ refund the faulty device. Knowing fully well the temperature in store is monitored and it will surely work.