Worst customer service ever

I have bought a rainbow home mattress.

Once we started using the mattress we woke up with head aches, stiff necks and back pain. The mattress is rated at 150kg weight limit (which for a queen means for each side), we both weigh less than 67kg, but from the first night it fell as if we are falling right through sleeping on no support. I contacted the company and they just said the mattress is fine and has no defects. They do not even try to establish the problem or assist. The mattress is advertised with a firmness level of 7, which is really firm supposed to be, but this is even worse than plush.