They used to come back after power cycling. I had them all doing auto-restarts once a week. If I unplugged one for some reason they usually came back without too much issue. They came back more reliably after setting them all up on static IPs. I have the NVR doing an auto-restart once a week too. When I noticed the outdoor cameras having particularly bad problems recently I switched all of the cameras to doing restarts everyday to see if that would help. It was the day after making that change that the two outdoor cameras stopped functioning.
I wonder if two Arizona summers were too much for these cameras. They were under cover enough that they never saw direct sunlight, but summers here can be brutal. I actually went with this camera system because it was one of very few I could find that said they would put up with higher temperatures. It hasn't been hot yet this year, but maybe the heat compromised something in the cameras and the humidity recently just finished them off.