How to turn off iCloud storage from my iPhone

Try this

Step 1: Free Up or Clear iCloud Space

If your storage is full, your @icloud.com email will stop receiving messages until you fall back under the limit.[1]

• Open Settings on your iPhone and tap your name at the top.
• Tap iCloud, then tap Storage or Manage Account Storage.
• Tap Backups and delete old device backups you no longer need.
• Go back to the main iCloud menu and turn off large sync features like iCloud Photos, iCloud Drive, or specific apps storing heavy data. 

Step 2: Downgrade or Cancel Paid Plans

If you are currently subscribed to a paid iCloud+ tier and want to move back to the free 5GB tier to avoid charges:

• Open Settings on your iPhone and tap your name.
• Tap Subscriptions (or tap iCloud / Manage Plan depending on your iOS version).
• Select iCloud+ and tap Downgrade Options or Cancel Subscription.
• Choose the free plan or lower tier and confirm with your Apple Account password.
• Note: Apple keeps paid storage active until the end of your current billing cycle. 

If you have "Optimize Storage" turned on, then all your stuff may not be on your phone, and you might lose information in turning off iCloud. See this:

Understanding iCloud Photos - Apple Community