First of all, storage in the phone itself has nothing at all to do with iCloud storage. Paying for increased iCloud storage doesn’t help the situation. This should shed some light in the difference between iCloud and device storage.
What's the difference between device storage and iCloud storage? - Apple Support
If you are constantly deleting content on the phone and it continues to do no good, you need to find out what is taking up all of the space on the phone.
How to check the storage on your iPhone, iPad, and iPod touch - Apple Support
If you still can’t resolve the constant full storage on the phone, it might help to backup the phone, restore to factory settings and then restore the backup.
Backing up the phone.
How to back up your iPhone, iPad, and iPod touch - Apple Support
Restore to factory settings.
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
Restore backup.
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support