Your iPhone may be taking photos in the "HEIF" format instead of the traditional JPEG format. While HEIF format is smaller than regular JPEG, it's also not compatible with a lot of computers.
Here's how you can turn it off:
https://www.coolmuster.com/idevices/disable-heif-image-in-ios-11.html
Also, if you have a Gmail account, I recommend downloading Google Photos, that app will automatically back up all your photos and you'll be able to access it from anywhere, whether it be your phone or computer.