This can be done, and I do this myself as my backup LTE is a hotspot box that has a LAN port that plugs into my firewall, so it is not USB based.
This is actually easier than you think, however you will need to contact support.
One of the gripes I've always had about the firewall section on the MX is they do not separate WAN1 and WAN2 firewall rules. They combine WAN1 and WAN2 into a single 'Outbound rules' section. They should separate them into two groups so that you have granularity control over it. Rumor is that they are planning on doing this in the future, but we'll see.
However, if you ask support, they can do something behind the scenes, so that the 'cellular' firewall rules are actually applied to WAN2
So when WAN1 goes down, the cellular failover rules 'append' themselves up, to the outbound rules above. So just think of the 'cellular failover rules' section as 'WAN 2 outbound rules'.
Here is an example of my setup (i took out any IP info to make it easier)