When you pay for storage, you are paying for iCloud Storage. You CANNOT add memory to an iPhone. Once purchased, the memory on your phone is fixed for as long as you own it. Read this to understand the difference between device and iCloud Storage --> What's the difference between device storage and iCloud storage? - Apple Support
All buying more iCloud Storage was to let you sync more data with iCloud, which you may not have needed to buy.
If you're out of memory on your iPhone, you have to delete things you don't need or use to free up space. Since you say you barely have any apps or photos, open Settings > General > iPhone Storage. What does it show for System Data all the way at the bottom? Also check all other categories as they populate. For example if you deleted photos, did you remove from the Recently Deleted folder?