L2TP over IPSec
L2TP traffic – UDP 1701
Internet Key Exchange (IKE) – UDP 500
IPSec Network Address Translation (NAT-T) – UDP 4500
Allows IPsec Encapsulating Security Payload (ESP) traffic to the VPN server is UDP 50
In General you can read here:
Do not forget that even if the server is still behind a NAT to behind a NAT, you must connect to the registry to register the key.