Hi,
When configuring the mobile interface in bridge or passthrough mode, most of the RUT240 functionalities become unavailable, including failover. You might be able to configure wired WAN as MAIN, then mobile in the bridge as second. The device should failover to mobile if the wired WAN becomes unavailable, however, the device will stay in mobile bridge mode until you set a static IP address on the PC connected to RUT240 to reach the WebUI and disable bridge mode.
Could you perhaps use NAT mode? This way you will be able to use failover.
Kind Regards,
Andzej