I've just purchased a Fritxbox Fon ATA 1020 and want to connect this
along with my wireless router (Buffalo G54 Airstation) to a cable
modem. My main computer will be connected via a network patch lead

The instructions say that the Fritzbox can be connected directly to
the modem or to a router - in which case it will "work like a computer
on the local network, and obtain its IP adress from the router". All
that is needed is that the Fritzbox is set to whatever function it is
performing.

But I thought it would be better to connect the Fritzbox directly to
the modem so that I can use the firewall and QoS features, and then
connect my wireless router to the fritzbox as a hub/access point
instead.

I had it working in this set up - the Fritzbox was set up as a router
and DHCP server, and assigned IP address to the Airstation and my PC
connected to it via a cable. The Airstation was set not to act as a
DHCP server.
But other computers could not access the internet wirelessly via the
Airstation.

Then, when I next turned on the PC, I had no internet access at all -
but the Fritzbox must have been letting the VoIP through, as my phone
was working.

Can anyone advise if this set up is possible, and what settings do I
need to st on the Fritzbox and the Airstation? Or should I just set it
up as per the instructions?

Cheers
dg