Paying for plenty of iCloud storage does not prevent an iPhone itself from becoming full because the two storage pools are separate.
Your iPhone has fixed physical storage used for iOS, apps, messages, photos, downloads and cached data. iCloud provides online storage for supported services such as backups, iCloud Drive and iCloud Photos.
Some iCloud features can reduce local storage pressure. With iCloud Photos and Optimize iPhone Storage enabled, for example, iOS can keep smaller versions of some photos on the device while full-resolution originals remain in iCloud.
That does not automatically offload everything. Large apps, games, offline video, downloaded music and other local data can continue taking up significant space regardless of the size of your iCloud plan.
Open Settings > General > iPhone Storage to see which categories are using physical storage. This screen can reveal whether photos, apps, messages or another category is responsible.
If iCloud has free space but the iPhone does not, upgrading iCloud again is unlikely to solve the underlying issue by itself. You need to reduce or optimize the data stored locally on the phone.
