All you will have to do is register the router with the STB.
Connect it all up with the STB, router and PC's powered off
Boot up the STB
Boot up the router
Boot up one of the PC's
Try to use IE or what ever browser you use and you will be asked to add the router to your list of devices.
You can also remove the two devices that are currently registered if you want to. However it doesnt really matter if you dont.
BTW
to the forum