in a browser type in the address bar "bthomehub.home" without quotes. This will take you to your routers web interface. Click settings and proceed to log in with the details.
When logged in, you will see more options.
Click advanced settings and then continue.
Click home network, then DHCP table (DHCP is the automated process of giving devices IPs). Click on your printers name from the list of devices on your network. Scroll down, check the box "always use this IP address". Apply. Take note of that address and your good to go.
Also, if it doesn't prompt you to enter a new password when you access it[(Its meant to the first time the router is accessed via web interface), and you don't know what the password is, then the default is the serial number (S/N) which is on a label on the actual HomeHub.
It should of been the serial number as default. If you reset the router by pushing a pin inside the little red hole on the side, it will put your password back to its default, and prompt you to change it when you first login to it. Don't worry, you wont lose any settings, and your internet and network will function as normal.