I bought more iCloud storage but my phone still says that my storage is full. What do I do?

You have bought more iCloud storage which is Apple's cloud storage system, it doesn't add storage to your phone and it isn't possible to buy storage for your phone.

Phone:

You can not add or buy more storage for your phone

iCloud:

You can buy more storage on iCloud. You might want to use this to save high resolution photos keeping the lower resolution versions on your phone to free up space. (Optimise Photo Storage)

You can buy more iCloud storage here Buy more iCloud storage – Apple Support