My client has a BT ADSL line and a router issuing a standard subnet 192.168.1.x / 255.255.255.0. I have a couple of pc's with addresses that cannot be changed, 192.168.10.123 & 124. Can I configure a VIGOR 2820 to allow the two pc's to access the internet through my clients router?

Any reason why you can't just change your clients router to use the .10 subnet instead?

Changing them makes most sense but you could set the second subnet address on the vigor to the .10 range and the give the router the gateway address they are "expecting" to see as a gateway.

Tries to think if that routes to internet... think it does.