Config problem with SoftEther VPN

I'm trying to setup SoftEther VPN server on a FreeBSD internet gateway (between my home LAN and the Internet).

My problem is exactly the same as in the thread "PC-to-LAN connection ok, but…" of softether users forum ( but I'm running FreeBSD, so the "tap" trick is not available:

- I can connect from an internet client to the public IP of the VPN server using L2TP/IPSec
- I get a LAN IP address provided by the isc-dhcpd service hosted on the VPN server
- I can reach other LAN machines


- I can't ping or access in any way services hosted of the VPN server (like DNS for example)
- I can't access internet through the gateway (VPN server is on this gateway, obviously)

I'm running softether-4.29.9680_1 from FreeBSD packages, on FreeBSD 12.0-RELEASE

Any hint or tutorial about this for FreeBSD ?