Good that the Beryl seems to ok. I think there are a couple of ways of approaching this. Full disclosure: I use my Beryl as a travel router and not as a file sharing device, so I’m hoping someone with actual knowledge will jump in.
If all you want to do is file sharing within your home network, then you can leave it in router mode. You’ll want to connect to the Beryl on its LAN side to manage it. On its LAN side it will be a DHCP server, but you don’t care about that. You can connect the WAN port to one of the switches, and its cabled connection will pull an address from the BT Hub DHCP server in the 1.64+ range. You don’t want that, so you can either change the Beryl WAN connection to static, and supply your 1.1 address on the WAN side, following the setup instructions, or you can set the BT Hub to hand out the same address to the Beryl every time it connects. I favor the latter, since helps avoid conflicts. Now go back into the file sharing configuration, and set the configuration to share files with WAN. Here WAN doesn’t mean the whole world wide internet, just the side of the router that is your 1.x network handled by the BT Hub. I think that will allow you to do what you want.
The other way would be to put the router into access point mode, which turns off the routing function and the DHCP server.
The first way may be easier, because you can unplug the travel router from your home network and the USB drive, and use it as a travel router without making any other changes. If you leave the USB drive in, just remember to change that WAN sharing back to LAN sharing.