1) Is your iPhone disabled? Does it state that it is disabled?
2) If your iPhone's storage is full then no matter how much external storage you buy, it will not increase your iPhone's internal storage space. The iCloud storage is designed to be used as a synch service and not additional storage.
3) You need to SAVE and create a backup of your iPhone first before you factory reset and not after. If your iPhone is disabled then you are not going to be able to create a backup before your reset your passcode or factory reset it.
Axel F.