Peguei o exemplo do PCC do wiki da Mikrotik e como tenho 4 interfaces com IP dinâmico fiz a adaptação abaixo.
Ele aceitou mas só consigo acessar as redes diretamente localizadas (que seja na LAN quer seja nas WANs). Para testar coloquei um cabo na minha máquina na LAN do RB750G e dois cabos da WAN ligados a um servidor de DHCP.
Eu desliguei o add default route das interfaces que pegam IP dinâmico. E retirei as portas 3,4 e 5 do slave da porta 2.
/ ip firewall mangle
add chain=input in-interface=ether1-gateway action=mark-connection new-connection-mark=ether1_gateway_conn
add chain=input in-interface=ether3-gateway action=mark-connection new-connection-mark=ether3_gateway_conn
add chain=input in-interface=ether4-gateway action=mark-connection new-connection-mark=ether4_gateway_conn
add chain=input in-interface=ether5-gateway action=mark-connection new-connection-mark=ether5_gateway_conn
add chain=output connection-mark=ether1_gateway_conn action=mark-routing new-routing-mark=to_ether1_gateway
add chain=output connection-mark=ether3_gateway_conn action=mark-routing new-routing-mark=to_ether3_gateway
add chain=output connection-mark=ether4_gateway_conn action=mark-routing new-routing-mark=to_ether4_gateway
add chain=output connection-mark=ether5_gateway_conn action=mark-routing new-routing-mark=to_ether5_gateway
add chain=prerouting dst-address-type=!local action=accept in-interface=ether2-local-master
add chain=prerouting dst-address-type=!local action=accept in-interface=ether2-local-master
add chain=prerouting dst-address-type=!local action=accept in-interface=ether2-local-master
add chain=prerouting dst-address-type=!local action=accept in-interface=ether2-local-master
add chain=prerouting dst-address-type=!local in-interface=ether2-local-master per-connection-classifier=both-addresses:4/0 \
action=mark-connection new-connection-mark=ether1_gateway_conn passthrough=yes
add chain=prerouting dst-address-type=!local in-interface=ether2-local-master per-connection-classifier=both-addresses:4/1 \
action=mark-connection new-connection-mark=ether3_gateway_conn passthrough=yes
add chain=prerouting dst-address-type=!local in-interface=ether2-local-master per-connection-classifier=both-addresses:4/2 \
action=mark-connection new-connection-mark=ether4_gateway_conn passthrough=yes
add chain=prerouting dst-address-type=!local in-interface=ether2-local-master per-connection-classifier=both-addresses:4/3 \
action=mark-connection new-connection-mark=ether5_gateway_conn passthrough=yes
add chain=prerouting connection-mark=ether1_gateway_conn in-interface=ether2-local-master action=mark-routing new-routing-mark=to_ether1_gateway
add chain=prerouting connection-mark=ether3_gateway_conn in-interface=ether2-local-master action=mark-routing new-routing-mark=to_ether3_gateway
add chain=prerouting connection-mark=ether4_gateway_conn in-interface=ether2-local-master action=mark-routing new-routing-mark=to_ether4_gateway
add chain=prerouting connection-mark=ether5_gateway_conn in-interface=ether2-local-master action=mark-routing new-routing-mark=to_ether5_gateway
/ ip route
add dst-address=0.0.0.0/0 gateway=ether1-gateway routing-mark=to_ether1_gateway check-gateway=arp
add dst-address=0.0.0.0/0 gateway=ether3-gateway routing-mark=to_ether3_gateway check-gateway=arp
add dst-address=0.0.0.0/0 gateway=ether4-gateway routing-mark=to_ether4_gateway check-gateway=arp
add dst-address=0.0.0.0/0 gateway=ether5-gateway routing-mark=to_ether5_gateway check-gateway=arp
add dst-address=0.0.0.0/0 gateway=ether1-gateway distance=1 check-gateway=arp
add dst-address=0.0.0.0/0 gateway=ether3-gateway distance=2 check-gateway=arp
add dst-address=0.0.0.0/0 gateway=ether4-gateway distance=3 check-gateway=arp
add dst-address=0.0.0.0/0 gateway=ether5-gateway distance=4 check-gateway=arp
/ ip firewall nat
add chain=srcnat out-interface=ether1-gateway action=masquerade
add chain=srcnat out-interface=ether3-gateway action=masquerade
add chain=srcnat out-interface=ether4-gateway action=masquerade
add chain=srcnat out-interface=ether5-gateway action=masquerade