I suspect it is because the two units are not coming online together before the "wan failure" happens.   Can you do this test for me, plug the cellular link into the spare MX and then see if that resolves the issue.  If it does, I suspect that if you pull the cellular unit back out it will keep working a expected.   If it does, then you might be able to try this (never tried it myself).  You are not going to have enough ports at the moment ... On the spare MX use the local status page and convert the first LAN port to a WAN port configured for DHCP.  Plug this new WAN port into the LAN side on your switches.  This will hopefully cause the spare MX to use the primary MX for its uplink.  No idea if it will work. Always wanted to know though ...   Otherwise you'll failover strategy will be to turn off the primary MX and turn on the second MX, and the reverse to fail back.  Still a valid as strategy as moving a cable from one unit to another (you could even just used a single power cable, and move the power cable between units). ... View more