If your backup is cellular then use the cellular firewall rules. https://documentation.meraki.com/MX/Cellular/3G%2F%2F4G_Cellular_Failover_with_USB_Modems#Cellular_Firewall_Rules_and_Bandwidth_Limits If your backup is WAN2 then you can raise a support ticket and ask to have the cellular firewall rules applied to WAN2.
... View more