Shortly i need to connect a Synology NAS to two separated networks. At default you can only define one Gateway for your Synology NAS. To define a second route, do this steps:
Activate SSH access to your Synology NAS:
configure first LAN interface:
configure second LAN interface:
configure default gateway:
Connect to NAS by ssh and configure the second route:
after configuring this, you can disable ssh access again.