You can use a pair of MX's in warm spare mode.   Two WAN circuits can plug into the first one.  You will only be able to use these in normal operation.  You wont be able to use the other WAN and 4G circuit.  They will only eb available in a failover scenario.   On the standby MX you can plug in the third WAN circuit, but note that it wont be able to be used unless the primary MX has completely failed (both if its WAN circuits and any 4G circuit would need to be down).   I would use a 4G router which has Ethernet ports in it, and plug in that to the second WAN port on the second MX.     Note that failover wont be invisible, because you are moving between different sets of IP address on different WAN circuits.  But it will be able to failover. Might take a couple of minutes in the worst case if moving between MXs. ... View more