iPhone can't upload photos to iCloud over WiFi, only over cellular data.

I have an iPhone 6s Plus 128GB (updated to the latest iOS 15.3.1) that refuses to upload my photos to iCloud over WiFi. My photos can only be updated using data from the cellular network. I have tried every solution method, even resetting the phone settings but nothing seems to work. I pay for a 200GB iCloud subscription monthly but it's frustrating that I can't efficiently upload my photos over WiFi. Instead, the Google Photos app works fine, whether it's over WiFi or cellular data, so the problem isn't caused by my WiFi connection. Also, my internet connection is through fiber optics so there should be no speed problems. I think it's an iOS bug on older apple devices.