Whilst it is a router its not really as feature rich as say a Cisco ISR. The BEFSR81 is designed to connect to a cable modem via its internet port (public side) and have up to 8 directly attached devices (private side), perfect for a small or home office. But the important thing to mention is that it’s really an internet router.
Whilst routers like these “could” be configured to perhaps to do what you want the main problems you come across, due to their intended purpose, is the inability to turn off NAT or firewall inspection rules. This is one of them
In order to configure a router to route between VLANs you need to either:
1. Have a router that is .1q aware. The router would connect to a configured trunk port on the switch and be configured itself with a sub-interface and IP address per VLAN. Traffic arriving at the router destined for another VLAN be routed out of that sub-interface. This is called router on a stick.
2. Have a router with as many ethernet ports as VLAN that it needs to route between. Each router port would have and IP address in each VLAN and route between them with a leg in each. Very port intensive!
3. Use a Layer 3 switch.
Hope this help,