You should start from https://help.mikrotik.com/docs/display/ROS/IPsec#IPsec-IKEv2EAPbetweenNordVPNandRouterOS
Also take a look to http://forum.mikrotik.com/t/static-default-route-im-missing-something/119183/2
and finally http://forum.mikrotik.com/t/mtu-troubles-using-ikev2-providers-like-nordvpn-work-around/135154/1