I don't mean to hijack the thread, but I am facing a similar issue (no 3g) and have not been able to resolve in a very long time.
I have the T-mobile original Galaxy Tab. I am using it on a different network (outside the US) though. I am running Sapir-10-20121207 (Android 4.1.2).I have tried both the eurotab (firmware P100XXJK5) and the t-mobile (T849UVJJB) modem.bin. With either firmware I get a cell signal but there is no data connection at all. Neither 2g nor 3g. It just doesn't get data. I double checked the APN info with the provider and it all matches.
I also tried changing the "APN type", but regardless of what I put there, if I go out of the APN setting screen and come vack it always changes back to "default,supl".
What could be the reason for no data?