There are dongles with limited range of IPs based on PROVIDER - MM78 has a lot of posts with discussion on providers...check them out I would say (the posts)
I had the same problem with limited IP pools when using AT&T.
Signed up for a plan with Verizon using their wireless jetpack. I have been using it for a couple months now. Logged around 250-300 IPs... no duplicates yet.