I use a Three network dongle, when I am in my caravan in West Cornwall and it works very well. But when I first had it, I couldn't connect to anything, despite it saying that I had plenty of credit and was connected.
I called the help line and a very helpful Indian chap put me right, straight away.
The point is that if the dongle says that it is in a good reception area, then a call to the help line should produce the goods.