[Q] I cannot connect to mobile data GTI9300

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http://forum.xda-developers.com – I've a I9300XXELLA Galaxy S III, it's home is UK, but I live in Egypt, I could receive calls, make calls and connect to internet through Wi-Fi and mobile network since then, I flashed a Supernexus ROM on my S III, still the making/receiving calls and connecting to internet through Wi-Fi work, but I can't connect to Mobile Data, the "H" in the upper bar doesn't exist though the mobile data symbol is already green (turned on) APN setting is correct, so I flashed firmware 4.1.2 official ROM, still the same. I don't really know how to deal with it, any help please? (HowTos)