Adjust the router at the other end-- maybe both ends. You could put the OBi in the DMZ at that end. You could just do specific ports instead.
For testing you may be able to plug that OBi directly into the modem. That will prove the concept before figuring out how to set things in the router. You may need to reboot that modem after disconnecting the router for your test.
See
http://www.obihai.com/FAQ.php# "I am not able to hear the far-end partys audio but they can hear me?"