Airport Extreme Card won't connect to 802.11n

I have a Mac Mini 3,1 with a b/g/n Airport Extreme card (Broadcom). I'm trying to connect it to 802.11n via a Billion 7404VNPX wireless router, without success.

The hardware spec says it's n-enabled, as does System Profiler and Network Utility. Still won't connect. Same problem with a similarly enabled card (Atheros) in my MacBook, whereas a 3rd-party (Edimax) PCI card in an ancient PowerMac G4 connects to n perfectly. I've forced the router out of g/n mode into n only. No go. Back to g/n mode and the 2 cards connect via g.

Any solutions available?

Interesting you said that. We have just grabbed a Billion 7800N modem/router and cannot for the life of me connect a MacPro 3,1 to it either. All our PCs connect no problem. I thought it was a problem with the mac but I dont think it is - I replaced the router for the old linksys G one I had and everythings connecting fine Mac wise , put the 7800N Billion to the same settings, channel, security etc, and still no luck. Anyone else had issues with Billion routers wifi and Macs?