Hello,
Could you please try upgrading FW to version 00.06.06.1 and check it again, and if possible reset router to factory configuration. I've just tried failover in all configurations, with WiFi as primary and as backup from Mobile and Wired networks it switched to backup on connection loss and back to Primary interface when it recovered every time.
BUT if problem persists could you please recreate the problem and send me troubleshoot file via PM so I can take better look at yours configuration and logs, but I hope it will not be necessary if you upgrade yours FW.
Best regards,
VidasKac