There is a trick to “humidity” levels. It’s generally relative humidity which has a temperature dependence. Take a plastic bag full of humid air at some RH value and heat it up and the RH drops. Relative humidity is how much water is in the air compared to how much water it can hold. As you heat air it can hold more water. So hot air at low humidity can still have a fair amount of water in it.
So while 10-15% RH is pretty good for filament storage for many people, that’s at room temperature. If your storage was at room temperature the humidity would be a fair bit higher. This is the part about dehumidifiers not really drying the air “a lot”. They dry some but aren’t expected or desired to really wring air out to dry as death valley.
You don’t say what kind of filaments you are working with (maybe I missed it) but PLA is pretty forgiving and can print well with higher moisture content than stuff like TPU or PETG. That’s just side information but something to keep in mind if you use them. The bigger issue seems to be long term heat and what it does to your filament.
Like @lion7718 said, you can dry your filament for a day or so but take it out of the hot box and put it in an impervious container for storage. And Ziplock bags are semi-permeable. It’s slow but over long time they approach no container at all.