I'm having the same issue. First, I've tried setting APN, it's ok. (same options, same phone model, same sim card - works)
Then, I reflashed Stock JBs, even the one with PIT file in it. Mobile data didn't work with any ROM.
I think the very official site for getting samsung fws is sammobile.com/firmwares (sorry, I can't post links)
There is only one thing I've not tried, reflashing stock GB. Does anyone think, it could help getting mobile data back?