It is unclear to me how your have your mAP set-up, is it behind another broadband router provided by your ISP? Because if thats the case you should start by forwarding the necessary ports from your ISPs router toward the mAP and if you use a non-default port it should be mentioned somewhere in the VPN-client-configuration, for example in the idea.
To give you an impression, my ISP gave a default router, I’ve set this up as a bridge and my hAP ac lite is setup as DMZ-host, so all traffic gets forwarded from the ISP router to my hAP. On my hAP I’ve setup an OpenVPN server using the official MikroTik documentation found on the wiki (https://wiki.mikrotik.com/wiki/OpenVPN#Server_configuration), since all traffics goes toward my hAP I’ve setup firewall on there as well, in the firewall I’ve configured the OpenVPN ports to accept new connections.
Since I get a dynamic IP from my ISP I use the RouterOS ability to preform Dynamic DNS (IP > Cloud) and use that address as my VPN-server. In the OpenVPN configuration I’ve setup the server name and the port I use.
I hope the way I’ve setup my OpenVPN can help you understand the steps you need to take in order to get your L2TP-connection from outside of your home network.