We have a customer that has more than one internet circuit. They want to use a cellular modem as their final failover if two other Internet Circuits. Could we configure a port on an MX85 to be an L3 port connected to the cellular modem and then have the MX failover to this device if the two Wan circuits go down.
Actually, this is likely not possible for the OP. Since Meraki has stopped certifying USB modems years ago, it's almost impossible to buy one that would work because all of the certified models are EOL. Updating the firmware to allow one of the RJ45 ports to be used as a cellular fail-over port would be a decent consolation for customers who buy MX devices believing USB cellular fail-over is still a realistic feature.