bad cellphone

Last year 2017 November bought Samsung Z4 cellphone at Jet store in Pinetown Sanlam centre, their advise me that it a good phone. within first month of using it the fone is freezing, I cant access whatsup it will keep on searching trying to connect for ever, and I cant download any apps, even from share it. it doesn't wana downloand any apps from unknown source. I feel like I wasted my money, as I had high expectations that Samsung is a good brand. I would advise anymore to buy Samsung cellphone especial entry level cellphone. I stuck now I don't know what to do,.. please help me.