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.