Using guest network feature behind a firewall

Hi, new here, but unable to find the answer to this problem I have with the RT-AC66U (probably not unique for this router by the way). Running .260 official firmware. To the case at hand:

I would like to use the routers guest network possibility, but I have a slightly more complex setup than what Asus reckoned with I guess. My setup is as follows:

Cable modem -> Zyxel USG200 FW port 1 (LAN1) -> LAN (192.168.1.x) with DHCP server -> RT-AC66U LAN-port
Zyxel USG200 FW port 6 (LAN2) -> RT-AC66U WAN-port (10.59.0.2)

When connected to the normal WLAN everything works fine, I can access all the other units on the LAN side and the Internet through the default gateway which is port 1 on the USG200.

When connected to a guest WLAN, 'nothing' works. I do not get an IP address from the DHCP server on my LAN. If I assign a static IP fot the computer I use I can ping the Asus, but that's it.

So, two problems I guess:

1. How to get my internal DHCP server (w2008r2) to assign IP-addresses to the clients on the guest WLAN
2. What static routes are needed to make the traffic on the guest WLAN be routed throught the WAN port on the ASUS, and not the LAN-ports.

Any help appreciated! ;-)