I upgraded but it still says low on storage

You cannot buy more storage for your phone. It will always have the amount it came with. You have bought space on Apple's synching service, iCloud. The only way to gain more space on phone is to delete photos, messages, apps, etc.. Be sure to save important photos and data to your computer or an external drive. This video may help...

You may need to activate optimization to make use of that extra iCloud storage:

Click on the link to learn how to optimize the storage on a device --> "How to check the storage on your iPhone, iPad, and iPod touch" - How to check the storage on your iPhone and iPad - Apple Support

- "If your device is low on storage, it automatically frees up space while installing an app, updating iOS or iPadOS, downloading music, recording videos, and more.

To make more storage available, your device can remove some of your items, like streamed music and videos, files in iCloud Drive, and parts of apps that aren't needed. It also removes temporary files and clears the cache on your device. But your device only removes items that can be downloaded again or that aren't needed anymore.

iCloud storage does NOT increase the amount of local storage on the device.