Old Photos/Videos Won't Load

This problem started a while ago. My older videos on my iPhone 11 pro would display a white circle with an exclamation mark in the bottom right, and the dreaded "An error occurred while loading the higher resolution version of this video" or whatever. (no trying to duplicate or edit them didn't help) I recently switched to the iPhone 15 and the problem persists. I tried turning "sync this iPhone" off, stupidly. now all my old photos except for around 400 have disappeared off of my iPhone 15. Although they remain on my 11 Pro, still not loading. It's just stuck at 'Downloading 3,140 originals'. In iCloud it says there are around 486 photos, and 204 videos. I have 2 years of photos/videos on my 11 pro that I hold dear to my heart. Please tell me if you have a fix for this.