just a note: my 3G dongle has an uSD slot, filled with a uSD with some data on.
An ok, there's no way to connect, neither with LuCI/Network/interfaces button.
I verified: this 3G dongle works fine with Ubuntu, but only if plugged before boot.
If plugged in with system on there's no way to turn on
just like with Yun. Myabe this information could help to solve the problem...