Hi exm,
I’m sorry to hear that the mattress you chose didn’t work out as well as you hoped for but it may end up being a blessing in disguise.
My first suggestion would be to completely reset how you are looking for a mattress and to read post #2 here (and the posts it links to including the tutorial).
You can also see some specific comments about the Vantage in post #2 here and as you can see I would avoid it completely because of the low quality materials inside it which would be a weak link in the mattress and if you can return your mattress for a refund and start all over again you would probably be much better off in the long term anyway.
Unfortunately it’s not possible to “diagnose” mattress comfort issues on a forum with any certainty because you are the only one that can feel what you feel on a mattress and there are too many unique unknowns, variables, and complexities involved including body type, sleeping positions, health conditions, and individual sensitivities and preferences that can affect how each person sleeps on a mattress in terms of “comfort” and PPP (Posture and alignment, Pressure relief, and your Personal preferences) or any “symptoms” they experience so a mattress that would be “perfect” for one person may be unsuitable for someone else to sleep on.
Having said that … there is more about the most common symptoms that people may experience when they sleep on a mattress and the most likely reasons for them in post #2 here.
The most common (but not the only) reason for lower back pain would be a mattress that is too soft and since you are still in your trial period the simplest solution would be to replace it with a mattress that is a better “match” for you in terms of PPP (Posture and alignment, Pressure relief, and your own Personal preferences).
I would avoid this mattress as well (or any Simmons or other major brand mattress) because it also contains some very low quality materials that are even lower quality/density than the Serta Vantage.
If you let me know your city or zip code I’d be happy to let you know about the better options or possibilities I’m aware of in your area.
Phoenix