Hello,
In order for failover to work, the RUT950 will have to be your primary router. I will work by switching between LTE and wired WAN, and the downstream devices will be able to see the difference between the sources.
I'm not sure what the Orange Box is, but if you'd like to simply pass the 4G connection to it, then it will have to support failover on its own, and theoretically, nothing needs to be configured on the RUT950, as long as it connects to LTE and the subnets do not overlap between it and the Orange Box.
More information about failover can be found here.
Best regards,
DaumantasG