How to retrieve back my iPhone photos that haven’t sync with iCloud?

I’ve taken photos and videos with my iPhone 12 without syncing with iCloud being some time due to no stable internet. During this period of time, I had rebooted my iPhone once, at first I didn’t realize some of my photos and videos were gone. When I would like to access them later, I only realized a period of my photos and videos (most probably from last synced to iCloud) just disappeared up to after rebooting my iPhone. This is so strange, where those photos and videos already took within local storage, it’s impossible for them just gone missing without syncing to iCloud yet my local storage is not full. May I know how can I get back those missing photos and videos?