You’ll have to figure it out for yourself. Because you have already configured multiwan on the router, it depends on these settings how you need to configure it. Otherwise my advice can break the whole work of the config. So look on the forum threads about multiwan there these settings are considered.
Or post the config, maybe some of the forum members will help.
PS http://forum.mikrotik.com/t/multiwan-with-routeros/163698/1