Phone problem

Yesterday I went to PEP van der Walt to query about my phone ,I bought a Hisense E7 on the 26 April 2019 so my phone is no longer working properly and I lost my slip but I do have the box of this phone and they told me they can't help me .I think is no fair because is been four months now they still have records of the phones that they sold. I need help on this query.