It seems when access to Google's servers is blocked (by a firewall for example) Photos doesn't display my photos. The photos are locally available and can be browsed with a file explorer app. I don't want all my photos stored in the cloud nor do I like the idea that Google is checking something every time I open the Photos app. Is there a workaround to disable this behavior?
Even thought the photos are stored locally, when in the main view of the photos app only the thumbnails or nothing at all (if thumbs are not cached) will be shown when internet is off/blocked/limited.
The only way you can access the local copies while using the photos app is to open the menu on the left and select "Device Folders" and then go into the Camera folder (or wherever it is you are trying to access).
Click Images to enlarge
Unfortunately because the internet is limited you wont have access to the nice new features such as the advanced photo search or the nice date order layout as these are all server-based features.
You can check if the photos are locally stored or not by:
Photos → Settings → Uncheck Show your google drive photos & videos
If your photos do not show now they were never on your device. To store them on your device download them from Google Photos and also turn off Backup & Sync if you don't want it.