Mattresses that sag

Bought a Cloud Nine mattress from them 30 Nov 2018. This was my 3rd mattress in 2 years and we decided to buy something decent, as previous mattresses always sagged. We were assured by the salesman that the cloud nine won't. 6 months later and my wife and I started experiencing slight back aches some mornings. In August we had had enough and I went to the store to try have it resolved. We were told that sag is natural to conform to your body contours. I told them that's why I asked for a bed that will not sag, yet here we are again. Still waking up with back ache some mornings. By the way, spot the **** compliment of the previous listing here.