Unsupportive mattress, non-responsive seller

I've had this mattress for about a year. Bought it brand new without trying it bcause New York Times Wirecutter recommended it. Cost $2500 with "discounts" (their mattresses are always discounted with a coupon code on their site). I've never slept through the night on this mattress. It's super soft, not even medium. I toss and turn all night to try to alleviate the lack of support for my lower back. I reached out to the salesperson about 6 months after purchase to ask about options to firm up the mattress a bit, in hopes of having better support for my back. He recommended I get a coir layer to put under the topper. So I did that (cost $150, also ordered from CP Rogers). That did nothing other than to cause shoulder pain as well. One year later, I'm out ~$2600 and CP Rogers doesn't respond to my complaints. I wound up ordering a new mattress, which will be delivered this week, from another retailer out of desperation. I really just want to stop waking up tired, with a stiff back. Note that I work out regularly and am in no way overweight.