Synology

Enable L2TP/IPsec Logs as debug:

vim /var/packages/VPNCenter/etc/l2tp/ipsec.conf

add or uncomment this parameters:

config setup
...
    plutodebug=all
    plutostderrlog=/var/log/pluto.log

Allow UDP 500, 1701 et 4500 on the NAS, don't NAT/transfert 1701 on router.