i was unable to get failover working in this manner, what i could get working was a load-balance between wifi-STA and mobile sim.
You need to set mobile WAN as your main (otherwise the modem sleeps) and then STA as your secondary.
Under load balancing, balance heavily to the wifi (10) and small (2) to the mobile WAN.
This worked quite nicely for me, very little data gets used over the mobile data.