Hello CohenOmar,
This page can assist with providing some suggestions for the photos being missing in the app: If you're missing photos or videos in the Photos app - Apple Support
In particular, since they're showing on the iCloud website try the steps in the "Make sure that iCloud Photos is turned on" section, amongst the others as well:
Make sure that iCloud Photos is turned on
iCloud Photos securely stores all of your photos and videos in iCloud, so that you can access them on all your devices. If you took a photo on your iPhone with iCloud Photos turned on but later turned the feature off, you might still be able to find a missing photo in iCloud Photos.
On your iPhone or iPad
1. Go to Settings > [your name], then tap iCloud.
2. Tap Photos.
3. Turn on iCloud Photos.
On your Mac
1. Go to Photos > Settings.
2. Click the iCloud tab.
3. Select iCloud Photos.
After you turn on iCloud Photos, you might need to connect to Wi-Fi and wait a few minutes. Then look for your photos again.
If you needed to re-enable iCloud Photos on the device, be sure to also connect the device to power in addition to a strong Wi-Fi network to help allow the sync process to continue.
Cheers.