I did nothing very special, just activate IPSec in the L2TP Interface config.
Here's my config to compare :
[gart@MikroTik] > /ip ipsec peer print
address=126.96.36.199/32 auth-method=pre-shared-key secret="nordvpn" generate-policy=port-strict policy-template-group=default exchange-mode=main-l2tp send-initial-contact=yes nat-traversal=yes proposal-check=obey hash-algorithm=sha1 enc-algorithm=aes-256,aes-192,aes-128,3des dh-group=modp1024 lifetime=1d dpd-interval=2m dpd-maximum-failures=5
[gart@MikroTik] > /ip ipsec policy print
src-address=X.X.X.X/32 src-port=1701 dst-address=188.8.131.52/32 dst-port=1701 protocol=udp action=encrypt level=require ipsec-protocols=esp tunnel=no proposal=default priority=0 ph2-count=0
[gart@MikroTik] > /ip ipsec proposal print
name="default" auth-algorithms=sha512,sha256,sha1,md5,null enc-algorithms=aes-256-cbc,camellia-256,aes-192-cbc,camellia-192,aes-128-cbc,camellia-128,3des,blowfish,twofish,des,null lifetime=30m pfs-group=none
Nothing more than the dynamically generated rules.
Maybe change the server at NordVPN, I was stuck at the begining during some days, I couldn't understand the issue because my config was really good... the issue was coming from the NordVPN server !!!