RouterBOARD RB2011U-Problem with interface eth3

Hello every one, I’m novice to routerOS programming and I’m facing some problems..
I have mikrotik routerboard and I want to give access to the internet to the lan that I’ve set up on interface eth3.
The pre configured eth2 interface is connecting to the internet, but the new ippool with the new dhcp server that I’ve set up on interface eth3 cannot. the pc on eth3 is getting the right ip address the right gateway and dns but it cannot go out the th internet..
My plan is to set eth4 and 5 as slaves ports to eth3 and then bridge the eth3 with wlan and put the appropriate firewall rules to the bridge.
But until now I have not succeed to set up just the simple one, which is to connect the interface eth3 with outside world..Do you have any suggestions?
Thank you in advance!