You can use eufy cams offline locally to some extent. I have the Indoor Cam 2k Pan & Tilt. Through the app you can go to storage and set it to NAS (RTSP). If you set it to continuous recording it will basically broadcast the feed through an rtsp stream. Unfortunately it doesn’t let you set a username or password for added security within your network. Though if you trust there’s no bad actors snooping around inside your network then it can be secure as long as you don’t open your firewall to the rtsp stream. Make sure you make the cams ip static on your router.
Once that’s set you can set a surveillance app of your choice up to monitor the stream. I use iSpy (free and open source) for windows (not their dvr agent, though that may work too) . If you go that route you can add a camera, choose FFMPEG(h264), enter the URL with the port added (i.e. rtsp://CAMSLOCALIP:554/live0), rtsp mode to auto and it should work. I went into the source options and adjusted “Resize to” to 1920x1080 for better resolution.
Mind you I mostly set it up to be going in a monitor nearby to see if my old dog needs help while I’m working. As such it doesn’t bother me much that a lot of the smart features don’t work in this setup. They still work, just not through the stream, and they still require internet. Not even sure I can rotate the cam without the app and the internet, though maybe there’s someway to do that in iSpy.
One issue I noticed is if your router, not your internet, restarts you may need to cycle NAS (rstp) on and off using the app to get the stream back up, which needs internet. Though in the scenario of your internet going down and using backup power for cams, router and PC it shouldn’t be an issue.