They each have a public IP? Static or dynamic?
I have some of those (sold under a different "brand") and I'm not surprised that at least some of them disconnect, as the quality is quite low. But, it's possible that the D-Link switch is defective. I'd test by replacing one of the cameras with a computer and see if it disconnects also (implying a faulty switch). Or connect a camera to the first switch--if it disconnects there too then probably it is faulty.
Sometimes resetting the camera to factory defaults will make it work OK, at least for a day or so.
I have some of those (sold under a different "brand") and I'm not surprised that at least some of them disconnect, as the quality is quite low. But, it's possible that the D-Link switch is defective. I'd test by replacing one of the cameras with a computer and see if it disconnects also (implying a faulty switch). Or connect a camera to the first switch--if it disconnects there too then probably it is faulty.
Sometimes resetting the camera to factory defaults will make it work OK, at least for a day or so.