Since the os7100 is taking to the sbc (and they're on the same subnet) it doesn't need a dg.

You will need to change your lan dg ip if it's the same ip as the sbc (eg make it 192.168.0.254).
Then change the os7100 dg to that.

Since both networks are the same range even if the os7100 had 2 lan ports you would still have to change the ip take on 1 of the networks as you can't have 2 lans on the same ip and subnet range on the 1 device.