I bought an iPhone 13 in cash converters,so it's not working properly,it freezing an slowly an it's always causing fluctuation

I bought an iPhone 13 in cash converters,so it's not working properly,it freezing an slowly an it's always causing fluctuation.,which resulted for me to take it back to the store,now the manager she's telling me that it's consumer fault there's nothing that she can do,an the phone that I'm taking about it doesn't have even one month,an she's said to me I must take it further she's not scared of any one,an I'm welcome to do anything.