I purchased extra storage and pay monthly for it. I still get warnings about storage being full!

Maybe you mistaken what iCloud Storage is?

iCloud Storage does not increase what you have on iPhone.

iCloud Storage are meant for backup and syncing.

To make space on your iPhone, you need to delete stuff you don't need or save important stuff onto your computer before you delete.

Does the warning say your storage is full on your device, or in iCloud? It's an important difference.

FYI, the storage capacity on your device cannot be increased. You subscribed to additional storage off-device, in the cloud.

Now that you do have extra iCloud storage, you can offload some items to iCloud to free up space on your device. See the article below for suggestions.

How to free up storage on iOS devices if you need more space for an update:

https://support.apple.com/en-us/HT203097 

You're welcome.