So if WAN is the main and SIM1 is the failover, kindly follow the below configuration.
Here mob1s1a1 is the SIM1. Saving this should make SIM1 as failover and we have the options for multiple failover.
Please note that the interface with the lowest matric value is the primary WAN and the next value is the failover. It can be changed by simply clicking & dragging the interface above in the same page.
Flush connections is by default blank, but its better to switch it to "Connected" as it flushes the DHCP leases in case of failover being triggered.
Hope this helps...!!