Bad mattress of zero quality

I bought a new base and mattress in 2019 November and in July 2020 I complained that their mattress is now sagging and very uncomfortable,they then changed it to a new one then in October 2020 I complained again about the mattress that they've send of which it's from the same company that it has also started to sag..then they gave me a number of their supplier for me to contact them of which I did but never got a response...then I went directly to the shop the manager was not there requesting that they give me back my money because it means even the mattress that they will give me will not be of good quality rather I go and buy myself the same cheap mattress that I had for years and it never gave me troubles they told me they will never give me back my money I rather add money and get another mattress and I told them I don't have money but they told me then they won't be able to help me ..please assist.