Scenario:
This scenario will show you how to configure a dual WAN setup on a MikroTik RouterBoard. We will configure two subnets, the first 192.168.1.0/24 subnet on ether3 will use ether1 as its WAN port whilst the second subnet 192.168.2.0/24 on ether4 will use ether2 as its WAN port.


Configure the router to accept DNS requests:
/ip dns set allow-remote-requests=yes

Configure your WAN ports for both subnets:
Network 1:
/ip dhcp-client add interface=ether1 add-default-route=yes use-peer-dns=yes use-peer-ntp=yes comment=wan1 disabled=no
Network 2:
/ip dhcp-client add interface=ether2 add-default-route=yes use-peer-dns=yes use-peer-ntp=yes comment=wan2 disabled=no

Add an IP Address to ether3 for the local subnet 192.168.1.0/24:
/ip address add address=192.168.1.1/24 comment=network1 interface=ether3 disabled=no

Add an IP Address to ether4 for the local subnet 192.168.2.0/24:
/ip address add address=192.168.2.1/24 comment=network2 interface=ether4 disabled=no

Setup your DHCP servers for both subnets:
Network 1:
[admin@MikroTik] > /ip dhcp-server setup
Select interface to run DHCP server on

dhcp server interface: ether3Select network for DHCP addresses

dhcp address space: 192.168.1.0/24
Select gateway for given network

gateway for dhcp network: 192.168.1.1
Select pool of ip addresses given out by DHCP server

addresses to give out: 192.168.1.2-192.168.1.254
Select DNS servers

dns servers: 192.168.1.1
Select lease time

lease time: 3d

Network 2:
[admin@MikroTik] > /ip dhcp-server setup
Select interface to run DHCP server on

dhcp server interface: ether4
Select network for DHCP addresses

dhcp address space: 192.168.2.0/24
Select gateway for given network

gateway for dhcp network: 192.168.2.1
Select pool of ip addresses given out by DHCP server

addresses to give out: 192.168.2.2-192.168.2.254
Select DNS servers

dns servers: 192.168.2.1
Select lease time

lease time: 3d

Configure your NAT masquerade rules for each Network:
Network 1:
/ip firewall nat add chain=srcnat out-interface=ether1 src-address=192.168.1.0/24 comment="masquerade network1 out ether1" action=masquerade disabled=no

Network 2:
/ip firewall nat add chain=srcnat out-interface=ether2 src-address=192.168.2.0/24 comment="masquerade network2 out ether2" action=masquerade disabled=no