Hi Jessy-1983,
Thanks for the clarification that you have not changed your Apple ID. Since you can see the photos on the iCloud website, but not on your iPhone, that tells us that all your photos are in iCloud and may have trouble syncing to your iPhone. We want to see if there is any status message on your iPhone for the iCloud Photos. To do this, go to Settings > Photos > look below iCloud Photos. Does it show any status message? If it says that you need more storage and you have Download and Keep Originals, you can save iPhone storage by selecting Optimize iPhone Storage. If there is no message, no worries, just work through these troubleshooting steps:
1 - Restart your iPhone with these steps: Restart your iPhone, iPad, or iPod touch
2 - Go to Settings > General > Software Update and see if there are any updates to install. If there are, be sure to backup your iPhone before installing them. Having a backup of your data is like having an insurance policy, you don’t plan on using it, but it’s great to have if you need it. If you need help, here’s how to back up your iPhone: How to back up your iPhone, iPad, and iPod touch.
3 - Once the backup is complete, please install the update. You can update your iPhone using either wireless over-the-air updating or update using your computer: Update your iPhone, iPad, or iPod touch.
4 - Make sure your iPhone is connected to the internet: Go to Settings > Cellular and make sure Cellular Data is enabled.
Also, go to Settings > Wi-Fi and make sure the the device is connected to a Wi-Fi network.
5. Check see if the device is in Low Power Mode. Go to Settings > Battery > If Low Power Mode is enabled, turn it off. If Low Power Mode is on, be sure to connect your iPhone to a charger so it doesn't power off.
Let us know if there is any change to your Photos after these steps.
Cheers