There's nothing wrong with your OBi. Just access it from the same subnet. There is no reason to be trying to manage it from another subnet. Your issue is caused by your own router configuration.
There's no reason to be concerned about using the OBiTALK portal to manage your device. As long as you are using a strong password, it's highly unlikely that anyone could guess your user name and password. To date, there has been no reported incident that I can recall, of someone hacking into an OBi device via the OBiTALK portal. If you want to use Google Voice with your OBi, you must configure it via the OBiTALK portal -- you can't do it via the local web interface.
Edit:
It occurs to me that you may have the "access from WAN" setting disabled on your OBi (which was what I think you were asking about all along).
I haven't tested this today, but this setting is on a page that isn't usually looked at, and I see that, by default, the device disables WAN access to the web server. Setting it up using the OBiTALK portal enables this access, so it's never been an issue for me. See the screenshots below. One is from the OBiTALK portal, and the other is from the local web server.