In your case you simply need to configure the router to operate in Passthrough mode. This mode will disable most of your device's functionalities and your router will pass it's mobile interface IP to the first connected device's interface or to the interface of a preconfigured device.
To set Passthrough mode, access router's WebUI, navigate to Network -> Interfaces. Edit mobile interface settings. In an opened configuration window set Mode option to Passthrough, below you can specify the MAC of the device to pass the IP address to.
For more details, please refer to this wiki page.