I ran into this issue last year with a set of Axis M1054 cameras. Four of eight M1054 cameras would go offline, same symptoms as you describe. Over a 2 to 3 week period, one would go offline, sometimes two. Cycling the port would restart the camera and get it back online. Usually they would not respond to ping when offline, but sometimes they would even though the web interface was down.
Network monitoring showed a really high rate of link errors for the four cameras, like 1,000 per day when we should have seen zero or one per week. Plus for some of the cameras packets were getting dropped but not consistently.
It turned out to be a combination of three problems.
(1) Some of the cable was bad when we re-tested it with a cable tester. That was replaced and the packet issue cleared up. The link errors remained. It may have been that the cable problems caused the connections to degrade to 10 Mbits from 1Gbits, as we experience that with some other workstation connections that had bad cable. The client software would connect, but getting recorded video from the server was slow as molasses in January.
(2) The system was originally running one VMS, and when the company was bought out their service dropped to zero, and we switched to Axis Camera Station software (it's a 50-camera system, no moitoring, just infrequent review of recorded video). It turned out that these cameras had not been reset back to factory defaults for the VMS change, and the previous VMS software had created settings inside the camera that were visible in the System Report, but were seen in the camera's setup pages unless you knew where to look for them or caught them in the System Report. The server names were no longer reachable, and this was the source of the link errors.
(3) We went months without an issue, then a camera went offline. We upgraded to the latest version of firmware at the time, because per the release notes there were "improvements to camera robustness" -- which we interpreted to mean. we fixed some bugs related to the camera going offline.
It's now been a year since we had the issues, and no problems.
If you have Axis cameras, do you see anything unusual in the System Report? Depending upon what the cause of the problem is, you may get some insight if you set up a syslog server and enable syslog on the cameras.
At the same site we also had an outdoor Axis P1347 that would go offline every few weeks. We found that the POE switch had unstable power, and replacing the switch solved the problem.