In modem mode the hub becomes, just that, a modem with all routing and NAT features disabled, thus disabling all LAN ports. The top LAN port becomes a WAN port (any LAN port on the Hub 3) so you can attach your own router to provide Routing, NAT and DHCP features.
You can only connect one device to a modem, but many to a router.
To switch the Hub back into router mode use http:\\192.168.100.1 as it looks like you already know, but I am still confused why you say you want the Hub in router mode which is the same as normal mode.