I'm having a problem setting up a Zyxel SSL10 appliance for "full" VPN

The SSL10 lives on the DMZ of a Zywall 5. I can sussesfully connect to
from the WAN side using the HTTPS login page and the JAVA SSL/VPN
application seems to run fine on my client PC. If I type an Ipconfig
on my client I do see a PPP ZyWALL-SSLVPN entry with a proper IP
address assigned (i.e an address in my target subnet). However I can't
PING or otherwise access anything in my target subnet.

I have noticed that the submask for the PPP ZyWALL-SSLVPN net is set
to where I would have expected a class C subnet, but
that might be irrelevant.

Anyway what can be wrong ? Do I have to do some further adjustment on
the Firewall itself ? I have looked at the logs (buth SSL10 and Zywall
5) and there is nothing obvious as far as I can tell.

Any help most welcome

Best regards