Late reply but might be usefull for somebody using this wimax device...
I had just same problem with same device : AirStream 1200 F23, and my pppoe connection settings for bridge mode were also wrong so no internet and public ip ether, here is the solution :
The device is still accessible at 172.16.1.1
But you have to setup your connection manually like this because there is no dhcp in bridge mode :
Address : 172.16.1.2
Subnet Mask : 255.255.0.0
Default Gateway : 172.16.1.1
Then you could access the device's admin web page, default login / pass is : admin / admin
Then change back setting to router mode
The crazy thing is that this usb modem device is a real router but there is no physical switch to reset it in case of problem !