1. Hello Can you use multiple cameras "offline" at all?
2. Do we need a 2n router at all or can we simply change it with a hub or a switch?
1. The camera will work when there is no internet connection. However, logically, you will not be able to access it over the internet if its internet connection is offline. So if you are using MyDLink, it will appear to be offline. And if you are using D-Link's dynamic DNS service to give your camera an externally-addressable (i.e. over-the-internet) URL, it will appear to be offline. You would need to access the camera via its local network (LAN) IP address, e.g. http://192.168.x.x:
port#. Of course that will only work if the device you are trying to access the camera from is also on the local network.
2. Presumably the gateway box provided by your ISP will still route local network traffic even if the uplink is offline. So no, you don't need a second router. However, my personal preference is to have the ISP box be just a passthrough device and to set up my LAN on a second router behind it, like FurryNutz describes and you may be doing already.