Sagging tempure mattress

In 2017 I bought a tempure cloud mattress which was a demo from the Menlyn store, I started sleeping on the mattress the following year and has serious problems with my back which I kept calling the Menlyn store to complain about however I was told that buying a demo means that I can get any exchange or refund. I fought hard through consumer rights and finally once they received a call from consumer rights it was only then that they paid attention to my complaint, they then offered me another demo which I was promised that it would perform better. The current bed that I have now is seems to be sagging in the middle and my partner complains that the bed is hurting his back , when I called to tell them I was told that I can’t be helped twice and that they won’t come out and check if the mattress has issues. The matress has a guarantee but they say I have forfeited it as I purchased a demo and not a brand new mattress. My experience with the mattresses/s as well as the after sales team has been terrible. All I want is for them to come take their matress and give me a refund as all the mattresses have not given me any joy or provide the comfort and support that they promise