How to properly organize a network 2 mikrotik and 3G Internet?

Question

Computer networks

There are 2 mikrotik devices: RB951 (is the central router of the network) and RB912 with a built-in 3G modem (recently replaced the USB modem). I would also like to organize a backup channel on the USB modem (initially it was planned to connect it to the RB912, but as it turned out, it has a USB modem and a miniPCI-E modem that can only work alternately. A reboot is required to switch).
There is a question as it is more correct to organize a network.
Now all network devices are connected to RB951, and it sends all traffic through NAT to RB912. He, in turn, sends everything to the Internet.
I would like to see who connects to the RB912 (now for him the entire network is like 1 device).
What and how would you recommend setting it up?

Answer 1

All network devices are connected to RB951, and it sends all traffic to the RB912 trunk, which NAT lets out.
For RB951, there will be 2 providers: the first is connected via usb (modem), and the second via ethernet (RB912).