RSLinx Classic cannot automatically browse a "remote" EtherNet/IP network like you describe. It can browse ControlNet and 1756 backplanes, but it doesn't have the ability to send out a broadcast packet on the Ethernet network that is connected between your CPU rack and your I/O rack.
But you can manually tell it the IP addresses of the devices on the CPU/IO network.
1. Navigate to the 1756-ENBT module on your laptop access network.
2. Browse the backplane and find the "CPU/IO" network's 1756-ENxT module.
3. Click on the [+] for that module to show the Ethernet network connected to it.
4. Right-click on the "A, Ethernet" network and select Properties.
5. Enter the IP addresses of the devices on the "CPU/IO" network that you want to browse.
That should allow you to browse through the 1756 backplane to the specific devices on the CPU/IO network using RSLinx Classic.