Hi bedtoosoft,
Unfortunately there isn’t a completely effective way to “fix” or “firm up” a mattress where the comfort layers are too thick or soft without removing and replacing the materials inside the mattress because a topper will still follow and sink into the softer layers or any soft spots underneath them. Toppers can be an effective way to soften up a mattress that is too firm but are not nearly as effective a way to make a mattress that is too soft any firmer. There are a few suggestions in post #4 here that may be helpful but unfortunately in most cases a firmer topper will only provide a partial or temporary benefit and may introduce other issues as well (such as a sleeping surface that is too firm or that doesn’t contour as well to your body profile).
I don’t have any personal experience with it and there is very little feedback about it other than on the manufacturer’s site but some of the other members here that may have used it will hopefully see your post and share their comments. It would be similar to a mattress protector or sheets that are very tight (often too tight and the source of complaints) which creates a “drum effect” on your sleeping surface which can firm up the sleeping surface but it may also introduce other issues because it may also interfere with pressure relief, contouring, or affect alignment. Their site does indicate that it has a 30 day money back guarantee so if there are no restrictions to the return policy and it is based only on whether you are happy with the effectiveness of the product then there would be little risk in trying it.
Phoenix