Purchasing the 2 TB iCloud storage option.. this does not expand the storage capacity of your device. You cannot expand the storage capacity of your phone.
Here is how to check what is taking up all your space
How to check the storage on your iPhone, iPad, and iPod touch - Apple Support
If the majority of you space is being taken up by photos, and you are using iCloud Photos.. make sure you have the Optimize Storage option turned on.
Set up and use iCloud Photos - Apple Support
Otherwise if you are not using iCloud Photos, you can transfer the photos to your computer and remove them from your phone.
Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support
My guess is that you don't actually need the 2 TB iCloud storage plan and you thought you were expanding your phone's storage capacity with it. You can check how much iCloud storage space you are using here:
Manage your iCloud storage - Apple Support