You will see the "!", if Photos cannot download your photos from iCloud. This can happen, if iCloud Photo Library has been enabled on the iPhone, with the option "Optimize Storage". Then Photos will remove the full resolution originals from the device and keep them only in oCloud. The iOS 11 upgrade enabled this behavior by default. You may want to disable optimize storage, if you did not turn it on intentionally. See this page in the iPhone User Guide: http://help.apple.com/iphone/11/#/iph961b96c4d
Optimize your storage or keep all your photos and videos in full-resolution on iPhone. Optimize iPhone Storage is on by default. It manages space on your iPhone by automatically keeping your full-resolution photos and videos in iCloud and smaller versions optimized for iPhone, as needed. To keep the full-resolution originals on your iPhone, go to Settings > [your name] > iCloud > Photos, then select Download and Keep Originals. Your originals are always stored in iCloud.
Download a full-resolution photo or video. If you’re not storing full-resolution versions on your iPhone, pinch to zoom in to 100%, or tap Edit. The full-resolution photo or video downloads automatically.
Make sure, that your iPhone is connected to power and fully charged, also connected to Wi-Fi. Turn off Low Power Mode in the Settings > Battery.
Then try to enlarge one of the photos with "pinch to zoom" to force the iPhone to download it from iCloud.
Is there a problem with your iCloud Storage? Are you running out of free storage?