Most routers have a DDNS client built in. You can use port forwarding on the router to gain access to the OBi unit directly, you will then have no restrictions when configuring the OBi.
I had been using this method, but it's not working for me anymore -- there are now XML errors when viewing most pages. Unfortunately, I don't know what change (firmware upgrade, network settings, etc.) caused this to start happening. Also, several other users have reported the same issue.
For me, it's not a disaster, as there is a Linux system left running at the remote site, and there is no problem accessing the OBi via SSH port forward or VPN. Worst case, I can remote desktop to the Linux box and run Firefox there.
If someone has taken the effort to track down what goes wrong, perhaps we can get Obihai to fix it.