Even after purchasing iCloud storage the device storage is showing full

Device storage and iCloud storage are 2 separate things. Buying iCloud storage does not in any way affect or modify your device's local storage capacity.

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

Hi there,

I'd start by reminding you that buying iCloud storage does not increase your iPhone storage.

However, it may help you to free up some space, by uploading Photos in the cloud for instance.

All you need to know to use iCloud correctly is here: Use iCloud on iPhone – Apple Support