Help on Simmons Ortho mattress

Hi Some really helpful comments on this site - thanks. But so many comments I am not clear what I need to do...so appreciate some advise. We have a Simmon Ortho 1400 mattress (4-5 years) on a slatted bed frame (2 joined frames running lenghtways). My wife and I are starting to get some lower back pains so want to resolve quickly and figure it's the bed. I checked and some of the slats are not sprung anymore, that gap is 5.5cm. So reading the reviews I am not sure if we should replace the slats and have some type of mattress cover over ther top or if we need to look at a more substantial change - bed and mattress?

I figure a bed frame 4-5 yrs old should be OK and also the mattress. Thanks for some suggestions. Tim
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