That is because you cannot just buy iCloud storage and expect it to fix your issue. Buying iCloud storage does not expand the capacity of your iPhone. You cannot expand the capacity of your iPhone.
About storage on your device and in iCloud - Apple Support
If you check your iPhone storage, what is taking up all your space?
How to check the storage on your iPhone, iPad, and iPod touch - Apple Support
If it's photos, then you can utilized the iCloud storage space that you bought to help save you some space.. just need to make sure you turn on the 'Optimize Storage' switch.
Set up and use iCloud Photos - Apple Support
if it's not photos taking up all your space, you may need to start deleting stuff or get an iPhone with larger storage capacity. Here is a user tip that can help you understand what iCloud is good for
iCloud Deconstructed - Apple Community
If you see that 'other' storage is taking up much space.. then review this (basically make sure you are updated to 13.6.1)
Clear Other storage on your iPhone or iPad - Apple Support