Been wondering the same thing, and as far as I can tell it's because your PirateBox has been configured with the IP address 192.168.1.1 during install. When you connect your PB to your router via ethernet cable, the PB works as a simple relay between your router and computer and isn't assigned an IP for itself. Your computer, on the other hand, does get an IP which probably isn't on the 192.168.1.x network. My router assigns IP addresses on the 192.168.0.x network, so I'm not able to communicate with the PB on the 192.168.1.x network.
If you want to communicate with your PB while it's connected to your router, you need to manually change the IP of your computer to be on the 192.168.1.x network, say 192.168.1.42 for example. Then you will be able to SSH to your PB, and navigate to piratebox.lan. You won't, however, be able to access the internet since you're no longer on the same network as your router.
AFAIK, there's no way (as of now) to have your PB work as a WiFi-extender and at the same time function as a PirateBox. In order to do this I think you'll have to make the PB work as a local DNS server, redirecting requests for the domain piratebox.lan to itself, while sending all other requests to the router and on to the wider internet. This would require changes to some configuration files, but I do not yet have any idea which files that would be.
Some would argue that functionality like this would undermine the entire concept of a PirateBox, since it's supposed to be "anonymous, offline filesharing, and the minute you connect your PB to an internet-connected router, it's no longer offline. I somewhat agree with that argument, but it would still be a nice function to have since it would allow me to connect such a device to any network and create local chatrooms and whatnot, while still being able to access the internet. It's always bugged me that, while at airports, in coffeeshops etc, I can't be connected to my PB and engage with people who connect and are curious about what this is all about while at the same time using the internet. I have to choose between being connected to my PB and hope that someone logs on and starts chatting, and connecting to the internet and being productive. Usually I end up connecting to the internet and just have the PB powered on just to discover some time later that several people connected and tried the chat, only to find no one to talk to.
If you want to communicate with your PB while it's connected to your router, you need to manually change the IP of your computer to be on the 192.168.1.x network, say 192.168.1.42 for example. Then you will be able to SSH to your PB, and navigate to piratebox.lan. You won't, however, be able to access the internet since you're no longer on the same network as your router.
AFAIK, there's no way (as of now) to have your PB work as a WiFi-extender and at the same time function as a PirateBox. In order to do this I think you'll have to make the PB work as a local DNS server, redirecting requests for the domain piratebox.lan to itself, while sending all other requests to the router and on to the wider internet. This would require changes to some configuration files, but I do not yet have any idea which files that would be.
Some would argue that functionality like this would undermine the entire concept of a PirateBox, since it's supposed to be "anonymous, offline filesharing, and the minute you connect your PB to an internet-connected router, it's no longer offline. I somewhat agree with that argument, but it would still be a nice function to have since it would allow me to connect such a device to any network and create local chatrooms and whatnot, while still being able to access the internet. It's always bugged me that, while at airports, in coffeeshops etc, I can't be connected to my PB and engage with people who connect and are curious about what this is all about while at the same time using the internet. I have to choose between being connected to my PB and hope that someone logs on and starts chatting, and connecting to the internet and being productive. Usually I end up connecting to the internet and just have the PB powered on just to discover some time later that several people connected and tried the chat, only to find no one to talk to.
