The Meraki devices only support two WAN/internet links, plus failover to either a USB cellular modem (or an inbuilt cellular modem in the case of MX67C and MX68CW).
You’ve a couple of options:
1. Use the inbuilt cellular modem instead of the MG21 - then your failover will work WAN1 -> WAN2 -> Cellular.
2. Depending on what your network design is then you maybe able to achieve something using tracked routes on the MX, but your preferred path will need to be on a LAN port. You can then go LAN1 -> WAN1 -> WAN2. You won’t be able to do SD-WAN to the LAN port. Remember the WAN1 port will need a path to the internet too. As I said, depends on your design as to whether this is practical or not.