Basically, I always assumed that the main router needed to know the different networks created by the Synology.
But it seems not. Maybe it does some NAT-ing with the internal IP Address.
Anyway, I opened/forwarded the ports, launched my VPN Client, and all traffic was going though the Synology and out on Internet.
so no need of adding static route in this specific case.
But I still don’t know how to do this in case I would need it.
JM