Make sure you are using all optimization settings. If you are and your iCloud storage is not full then upgrading iCloud storage won't help.
Click on this link to "Learn the difference between your device and iCloud storage, ways to manage your space, and where to buy more if you run out." --> https://support.apple.com/HT206504
Also discusses optimization.
Click on this link to read how to 'Manage your iCloud storage' --> https://support.apple.com/HT204247 - including seeing how much storage you have and how to delete items stored in iCloud such as backups, photos, files, mail, messages and voice memos
It might also help to just check how your storage on your phone is being used:
How to check the storage on your iPhone, iPad, and iPod touch -https://support.apple.com/HT201656.
One thing people find is "other" can take up a large amount. There's not too much you can do about "other" except to maybe change the apps you use. Here's information I can find about dealing (or not) with "other" storage on i-devices:
How to check the storage on your iPhone, iPad, and iPod touch - Apple Support - http://support.apple.com/HT201656 - "About cached files in Other ... When you stream music or video, that content is stored as cached files on your device so you can quickly access it again. Your device automatically removes cached files and temporary files when your device needs more space." "You don't need to delete them yourself." (Which I interpret to mean you cannot delete them directly yourself.)
How to Clear "Other" in your iPhone's Storage - Use Backup
https://discussions.apple.com/docs/DOC-250002774
- A somewhat drastic but oft mentioned solution achieved by making a backup of your device which will not include "other", erasing the device, then restoring the device from your slimmed backup.
Clear the history and cookies from Safari on your iPhone, iPad, or iPod touch
https://support.apple.com/HT201265
What is "Other" and What Can I Do About It? - https://discussions.apple.com/docs/DOC-5142
Check the cellular data usage on your iPhone and iPad - https://support.apple.com/HT201299
- "To see the cellular data usage for individual System Services, go to Settings > Cellular or Settings > Mobile Data. Then scroll to the bottom of the screen and tap System Services. Cellular data can't be turned on or off for individual System Services."