3G Internet problem

I just ran into a problem and I am too stupid to solve it. Maybe someone out there can help me.

I want to use a 3G USB modem as backup internet connenction or as regular internet connection. I found a very handy router/modem (dovado tiny) for this case as you can not connect such a USB modem direct to UT. You can use it either as router or bridged as modem.
Both possibilities work fine with UT.

But - and there I am lost - in both cases I can not connect to the UT webinterface from the outside.
This I understand when I use the dovado tiny in router mode: Double-NAT problem.
But when I use the dovado tiny bridged as modem I have the same issue. :mad:

This seems to be a general Problem with 3G connections. If I connect the USB modem direct to my Notebook I have internet access with an external ip adress 77.116.247.233.
If I check the LAN settings I find as IP adress 10.38.109.233, standardgateway 10.38.109.234, dhcp Server 10.38.109.234, dns Server 213.94.78.16.

If I connect via UT and the USB modem the situation is the same. I get an external IP and a different one on my WAN interface.

Please help me to understand and what can I do to use the USB modem? :confused:

thanks
MIB