Just create a warm spare pair as described on https://documentation.meraki.com/MX/Deployment_Guides/MX_Warm_Spare_-_High_Availability_Pair You only need one license for the MX pair as the warm spare will only be passive. I would suggest using a small switch and connect both MX WAN1 to your WAN provider. Then, use your first MX to connect to the cellular network. Failover behavior is described here: https://documentation.meraki.com/MX/Deployment_Guides/MX_Warm_Spare_-_High_Availability_Pair#Cellular_Failover_Behavior (in a nutshell: if WAN1 on MX1 fails, it will fail over to WAN1 on MX2. If that also fails, it will automatically fail over to your cellular connection). As for the uplink IPs: you can choose to have a „cluster IP“ that can be used to make routing to your internal networks easier. The downside is that you‘ll need a third IP address on the transfer network to your ISP which is perhaps not possible in your setup. The other option is using two separate IPs for both MXs and leave out the „Virtual IP“ completely. You would have to adjust the routing from your ISPs Routern to your internal network manually when MX1 goes down for whatever reason though.
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