It sounds as if you have the OBi302 (is it really a OBi302 as oppose to the OBi202?) after the main router. Since the OBi302 has router capability, have you try putting the OBi before the Asus AC66U router and have the Asus in bridge mode? In this setup, the Asus will simply be acting as a switch with Wireless capabilities. If you create this setup, are you still having the problems you describe earlier. Which VOIP provider(s) do you currently have setup on the ATA? If you lower the volume on the phone, does the echo still exist? Trying starting over from scratch by resetting the OBI to default and test.
With the Asus in place, are you having the echo and dial tone problem currently?
Maybe you should open a ticket with OBi Support to see if they can diagnose the problem for you. Why did you purchase a OBi302 versus the OBi202? The OBi302 is primarily geared towards VOIP providers, with no native google voice capability.