can't see public ip on split connection externally

We have an ADSL connection going to a Netgear Router (I'll refer to this as the primary router). This router then splits all incoming connections to 3 different networks, all with their own public ips. The setup is we have one netgear router (the primary) acting as a splitter, which picks up the net connection. The firewall rules are then configured to forward all incoming connections to 3 more routers (which I'll call secondary), which then feed the 3 networks. One we use for a remote backup server, the other is our workshop connections, the 3rd is spare. Each of these secondary routers has it's default gateway set as the primary router.

The problem is that the secondary router that feeds our remote backup server recently crashed and burned (quite literally), so we've replaced it with a new identical router. However, wheras before we could connect to the remote backup server using the public IP of that secondary router, after replacing it I can't ping it's public IP address or use remote desktop connection to access the server. I can however access the server by going to the public IP of the primary router. I can't work out why this is the case, as I though that the primary router would not know where to send the incoming RDC port as it forwards everything to all 3 secondary routers.

What I need to identify is why I can't see the public IP address of the secondary routers externally. How should I be configuring this setup? Hope that all makes sense!

Thanks!
Sam
Teknite