Unfortunately, purchasing more iCloud storage doesn’t do what most people hope it does. While having some additional iCloud storage can help free up some space on your device, you cannot use iCloud for off device storage. iCloud is a syncing service Only for the most part.
About storage on your device and in iCloud - Apple Support
This is worth a look as well. There are some suggestions that might help.
How to check the storage on your iPhone, iPad, and iPod touch - Apple Support
The bottom line is that the physical storage on your device cannot really be supplemented and it is a fixed amount. The only way to get storage back is to free up space by deleting content.