Hello,
Thank you for your question.
By default router always searches for internet connection, in the default policy rule you can see that router searches for connection in WAN then in mob1s1a1, then in mob1s2a1 interfaces. If all interfaces have internet connection then router chooses interface with highest priority (you can set priorities in WebUI > Network > Interfaces by dragging and dropping interface you wish to change priority).
A failover Interface Configuration page is used to configure how the device will determine whether an interface is online or offline. By enabling it your chosen interface is always checking if it has internet connectivity, when the main interface loses connection to the internet then router starts using another interface with highest priority.
NOTE: in RUTX12 both SIM cards are active at the same time
Let me know if this information helps you.
Kind regards,
Edvinas