Enable L2TP IPsec VPN on a EdgeRouter X:

1. SSH to EdgeRouter.

configure

set firewall name WAN_LOCAL rule 30 action accept
set firewall name WAN_LOCAL rule 30 description ike
set firewall name WAN_LOCAL rule 30 destination port 500
set firewall name WAN_LOCAL rule 30 log disable
set firewall name WAN_LOCAL rule 30 protocol udp
set firewall name WAN_LOCAL rule 40 action accept
set firewall name WAN_LOCAL rule 40 description esp
set firewall name WAN_LOCAL rule 40 log disable
set firewall name WAN_LOCAL rule 40 protocol esp
set firewall name WAN_LOCAL rule 50 action accept
set firewall name WAN_LOCAL rule 50 description nat-t
set firewall name WAN_LOCAL rule 50 destination port 4500
set firewall name WAN_LOCAL rule 50 log disable
set firewall name WAN_LOCAL rule 50 protocol udp
set firewall name WAN_LOCAL rule 60 action accept
set firewall name WAN_LOCAL rule 60 description l2tp
set firewall name WAN_LOCAL rule 60 destination port 1701
set firewall name WAN_LOCAL rule 60 ipsec match-ipsec
set firewall name WAN_LOCAL rule 60 log disable
set firewall name WAN_LOCAL rule 60 protocol udp

set vpn ipsec ipsec-interfaces interface <id>

set vpn l2tp remote-access ipsec-settings authentication mode pre-shared-secret
set vpn l2tp remote-access ipsec-settings authentication pre-shared-secret <secret>
set vpn l2tp remote-access authentication mode local
set vpn l2tp remote-access authentication local-users username <username> password <secret>
set vpn l2tp remote-access client-ip-pool start 192.168.100.240
set vpn l2tp remote-access client-ip-pool stop 192.168.100.249
set vpn l2tp remote-access dns-servers server-1 <address>
set vpn l2tp remote-access dns-servers server-2 <address>
set vpn l2tp remote-access outside-address <address>
set vpn l2tp remote-access mtu <value>

commit ; save

Source: https://help.ui.com/hc/en-us/articles/204950294-EdgeRouter-L2TP-IPsec-VPN-Server