MikroTik L2TP

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Hello guys: I'm looking for an expert on MikroTik to assist with L2TP VPN setup. We're already running PPTP but need to enable L2TP for iOS/MAC users. We tried http://forum.mikrotik.com/viewtopic.php?t=111772 without success. I can see the connection starting; but it disconnects. Suspect encryption issue. MikroTik is behind a firewall NAT with UDP ports 500, 1701 and 4500 and the IP-ESP protocol, which is IP protocol 50; ESP configured to internal IP of MikroTik.
Kindly inbox me with your quote. Thanks