Unlike DH+, ControlNet, DeviceNet, etc, you can't automatically browse through a ControlLogix backplane to a remote IP network.
You have to configure RSLinx Classic to connect to devices on the "remote" network.
Browse over the Ethernet driver to the 1756 chassis, then to the "remote" 1756-ENBT, then select the Ethernet network underneath that 1756-ENBT object in the network browse tree.
Right-click and select Properties, then select the Browse Addresses tab. That's where you enter the IP addresses of the remote devices.
That's for RSLinx Classic. I believe that RSLinx Enterprise will actually automatically browse the remote network, but I don't have a system handy to test that.