How do I install Untagle's firewall with an Asus router as the access point?

Hello peers,  

I am setting up the Untangle's firewall in router mode using an Asus router as an access point. The configuration is Internet to external WAN, switching the Internal LAN interface, and switching the access point's LAN. The external WAN of Untangle is set up to auto DHCP. 

My question is which IP address should I use for the access point in order to set up the range in the Untangle external WAN? As of now, the Asus access point has 192.168.50.2, mask 255.255.255.0, and default gateway 192.168.50.1.

Thank you for your help.