@retsaw
I can't set the DIR855 as a client bridge so my wired computers will have no way of connecting. The good thing about tethering to the N900 was that all my wired connections will instantly have a connection. As if nothing had changed on my network.

@9000
yep, you are correct. Though it's not really a wifi connection between the router and N900 but a USB one.
It just doesn't work though, I'm not entirely sure why. My WAN status shows as disconnected on the OLED display before I even try and connect through any LAN computers to the internet.

Anyway, my new ISP activated in the evening so at least I have broadband again, but it would be nice to get this working for future reference.