I buy a space but still shows me that iPhone storage full

You cannot buy more storage for your phone - it will always have the amount it came with. You can buy space in iCloud, Apple's synching service. To create space, you must delete photos, messages, apps, etc. Be sure to save important photos and data to your computer or external drive. Read this ...

What's the difference between device storage and iCloud storage? - Apple Support

You are dealing with two different kinds of storage. You need to learn how to use iCloud settings on your device to better manage the fixed space on your device.:

What's the difference between device storage and iCloud storage? - "Learn the difference between your device and iCloud storage, ways to manage your space, and where to buy more if you run out." Click here --> https://support.apple.com/HT206504

No.

You rented (monthly) extra iCloud storage and it does not increase the storage capacity on the device.

I purchased more I cloud storage for my I pad mini it said my purchase was successfully completed. I have no seen any increase in my storage? What went wrong?

Did you read all the posts in this thread?