Offload photos
How do I remove photos from my ipad without deleting them from The cloud and my other devices?
iPad Pro 9.7-inch WiFi, Cellular
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If you are using iCloud Photos, you cannot delete the photos from the phone without deleting them everywhere including in iCloud.com. iCloud is a syncing service not an off device storage facility.
You can archive the photos to a computer and then delete them from the phone. Read this for instructions.
Archive or make copies of the information you store in iCloud - Apple Support