I found another approach using mangle rules, to capture the traffic going from your connected IPs (router and LAN) to the not connected IPs:
/ip firewall address-list
add address=192.168.1.0/24 list=Connected # WAN network
add address=192.168.100.0/24 list=Connected # LAN network
add address=192.168.100.0/24 list=LAN
add address=255.255.255.255 list=LAN
/ip firewall mangle
add action=mark-routing chain=output src-address-list=Connected dst-address-list=!Connected new-routing-mark=route1 passthrough=no
With this code, I assume you have some route marked as "route1".
Hope it helps.