This is likely due to the modem/router acting as the default gateway for all of the devices on your network. It is also likely providing DHCP services (assigning addresses to the computers). So, when it goes offline, the computers may lose their leases and then be unable to communicate on the network. See if you can configure the DHCP service on the modem/router to have a longer lease time. This will help when the modem/router goes offline and the local computers need to talk to each other. This is not a function of having an unmanaged switch in the network.