Hi nael,
You can see some pictures and comments about the Sultan Hallen in posts #5 and #6 here.
Unfortunately your mattress is a lower budget mattress that uses lower quality and less durable materials and components and the most likely reason for the issues you are having is because the lower quality/density polyfoam and the fiber in your mattress that is above the coils have softened and are breaking down. This would lead to soft spots in your mattress and would make it more likely that you will feel the innersprings under the padding. It’s much less likely that your issues are because of the mattress cover.
While a topper can certainly help to soften up a mattress that is in good condition and has no soft spots or sagging in the sleeping surface and just needs some additional softness or pressure relief … it’s much more difficult to firm up a mattress that is too soft or that has developed soft spots or is sagging because a topper will generally just follow any sagging or soft spots in the mattress underneath it. At best a firmer topper may provide a partial or temporary solution and at worst it could make things worse (it could make the top layer you are sleeping on too firm in terms of “comfort” and pressure relief and the layers underneath it may still be too soft to provide good support/alignment under the topper. While they may not be completely effective … there are some suggestions in post #4 here that may be helpful.
If you do wish to try a topper to help isolate you from feeling the innersprings then Ikea sells the Tromsdalen latex topper which is a good quality material and some of the sources in post #21 here also sell toppers that can ship across Canada but I would keep your expectations of success reasonable because they may not provide the firmness you are looking for on top of a mattress that is softening and breaking down.
Phoenix