Have you tried running netstat -r in terminal when connected to this wifi(you don't have to be root for netstat). You want to see if there is actually any gateway ip address listed. If there is no listing with a gateway ip address, there is no way for you to be routed to their public ip.

If there is no gateway, then something is happening when their dhcp server tries to pass this to your phone. You could ask the network guys if you could use a static address and configure the ip, DNS, and gateway addresses, through the advanced settings, though they probably won't go for it.

You could also add a default gateway, get the address from the network guys, without having a static address, using the route command in terminal, though you need root for this, and you should read a little on the command and the options before doing this.