Make sure to update to the latest version of Fully Kiosk Browser as the newer versions have an option to Allow Mixed Content as noted in the thread @Sgt.Flippy_PJ linked to:
It’s possible that two of your devices automatically updated their Fire OS version. Amazon bundles updates to the embedded browser (‘webview’) in their Fire OS updates and that could be impacting your tablets.
Updating to a newer Fully Kiosk version and setting the Mixed Content Mode to ‘Always Allow Mixed Content’ will likely resolve your issue.
If you are using a local IP address to access your cameras, you may need to use the *.nip.io URL format mentioned in the post that @Sgt.Flippy_PJ linked to above since Chromium 111 and newer introduced a bug with the ‘Allow Mixed Content’ feature and local IP addresses. (The details of this are in the linked thread)