I have multiple WANs and regadless of the actually active WAN I redirect my backup WAN sessions with something like that:
/ip firewall mangle
add action=mark-connection chain=input comment="Attach a marker wan1 to every session initiated from the WAN1 side (bridge1)." disabled=no \
in-interface=bridge1 new-connection-mark=wan1 passthrough=no
add action=mark-routing chain=output comment="All sessions originating WAN1 will get answers back through the same interface." connection-mark=wan1 disabled=no new-routing-mark=\
back passthrough=no
add action=mark-connection chain=prerouting comment="Attach marker wan1 to every session originating WAN1, but what are redirected through the router to some internal host." disabled=no \
in-interface=bridge1 new-connection-mark=wan1 passthrough=no
add action=mark-routing chain=prerouting comment="All sessions originating WAN1 will get answers back through the same interface." \
connection-mark=wan1 disabled=no new-routing-mark=back \
passthrough=no
/ip firewall nat
add action=src-nat chain=srcnat disabled=no routing-mark=back src-address=\
192.168.0.0/16 to-addresses=WAN1_IP_ADDRESS
/ip route
add comment="WAN1 connection using hosts use that default route." disabled=no distance=1 \
dst-address=0.0.0.0/0 gateway=WAN1_ISP_GW routing-mark=back scope=255 \
target-scope=10
Note: I hope I didn't left anything important out of it.