Using Draytek 2920 as a WAP behind a firewall

Hi,

We utilize a redundant Vigor 2920 as a WAP for our internal network.

We also have a Guest wifi network using a seperate Netgear WAP connecting to our backup adsl line.

We are now removing our backup adsl line, so would like to add the Guest wifi to the Draytek and have it handle both.

I have setup the vlans and Lan2 settings on a seperate subnet.

Our firewall on the internal network is the D.GW

I have maxed out the NIC's on the firewall so i cant add another interface in for a 10.0.0.0 network.

How can i make the Draytek do its routing with the vlans in place so the guest wifi network on 10.0.0.0 forwards its traffic to the firewall on our 192.168.1.0 network so it can get out onto the google machine?

Thanks