May i know what configurations you made for pass through mode ?
What is the device you assigned the WAN IP address to ?
What is the IP address you are having from ISP is it public ?
How are you having the full modem access through Mobile IP ? SSH ?
Can you send a ss for the remote http access of the device in System>Administration>Access Control?
Have you tried to upgrade the device firmware to the latest ?
Pass through mode disables most of the device's features , if you are trying to reach the device by Mobile IP and you assigned the Mobile IP to some other device , then you won't be able to access that device.