This sounds like your iphone has reached the iCloud storage limit.
If you exceed your iCloud storage limit, photos and videos won't upload to iCloud and your library won’t stay up to date across your devices. To continue using iCloud Photo Library, upgrade your storage plan or reduce the amount of storage that you're using.When you sign up for iCloud, you automatically get 5 GB of free storage for Mail, Backup ,iCloud Photo Library, and iCloud Drive. Your purchased music, movies, apps, books, TV shows, and shared albums don’t count against that free storage. If you need more iCloud storage, you have two options:
If you have an annual plan, you can upgrade or downgrade to one of the monthly plans above only.
Upgrade your iCloud storage plan
You can increase the amount of storage by selecting a new plan from your iPhone, iPad, iPod, Mac, or PC (for example, a 200 GB plan to replace a 50 GB plan). Your existing storage plan will be canceled, and you'll be charged the prorated cost for the upgrade (the cost of the higher-tier plan minus the prorated cost of the previous plan). Your plan is upgraded immediately, and the new monthly payment date is updated to reflect the purchase date for the new plan. You'll be notified by email each month about 7 days before the monthly payment for your storage plan is due. Your storage upgrade is billed every month unless you cancel by downgrading your storage plan.
iPhone, iPad, or iPod touch:
If you're out of storage, your devices won't back up to iCloud, and new photos, videos, documents, and data won't stay up to date.